Fuel Efficiency and Slow Driving? 1114
vile8 writes "With the high gas prices and ongoing gas gouging in my hometown many people are trying to find a reasonable way to save gas. One of the things I've noticed is people driving exceptionally slow, 30mph in 45mph zones, etc. So I had to take a quick look and find out if driving slow is helpful in getting better mileage. I know horsepower increases substantially with wind resistance, but with charts like this one from truckandbarter.com it appears mileage is actually about the same between 27mph and 58mph or so. So I'm curious what all the drivers out there with the cool efficiency computers are getting ... of specific interest would be the hemis with MDS; how do those do with the cylinder shutoff mode at different speeds?" Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?
Fuel economy (Score:5, Informative)
I spent some time researching this matter after a discussion at work started about it.
Something that I had observed in my car was that my fuel economy increased as my speed increased.
At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg. And at 65, i got about 20mpg. This testing was done along I-10 between Jacksonville and Los Angeles. There's lots of room to set the cruise control. A test usually consisted of fueling up, then a hard acceleration to the testing speed and setting the cruise control to handle maintaining the speed for the next 300 to 350 miles. Individual tests were spot checked (repeated somewhere else on the drive).
In researching this, it wasn't a matter that my car is "faster", stronger, or just plain cooler. It's a function of the drag of the vehicle and the RPMs the engine is turning.
Most cars make their best fuel economy somewhere between 1800 to 2200 rpm. Ah ha! My car has a 6 speed stick. If I'm in 6th gear it's turning about 2000rpm at 85mph.
I then compared ground speed to engine speed ratios of other cars, partly selected because they were owned by people in the discussion, or because they were fairly common cars. Depending on the vehicle, it's best cruise speed could be anywhere between 45mph to 90mph.
Re:Fuel economy (Score:4, Insightful)
I can confirm that in my Saturn, optimal highway fuel efficiency is 55 to 60mph. I've tested this quite extensively. If you follow hypermiling discussions, for most people, their experience is quite similar. If I drive my Saturn at 80mph, I get about 30mpg. If I drive at 55-60mph, I usually get just over 40mpg. On a good trip, if I combine it with shifting into neutral for downhill runs, follow large slow-moving vehicles (no, not tailgating; I always keep a safe distance), and so forth, I've gotten 45mpg out of it. This is repeatable and has been determined over dozens of documented fillups.
In city, I haven't been able to collect good data about whether my city hypermiling techniques are helping significantly or not because my partner does most of the city driving on the same vehicle, so it messes up my numbers. I don't do the dangerous things like shutting off the engine or doing breakneck turns, but I do accelerate slowly, look way ahead and take hills into account, coast to red lights, time lights, take turns at moderate speeds, and avoid roads with stop signs. Given that I use my brakes only a fraction as much, I *should* be getting significantly better mileage, but unfortunately, I have no way of knowing.
Re: (Score:3, Insightful)
Or wishful thinkers, yeah. Given the physics of the thing, it's pretty darn unlikely, and getting more unlikely the higher speeds are claimed as "optimal".
Wind-drag is by far the dominating force in high-speed level driving, and it goes up significantly faster than linear, typically with the speed squared.
Your engine is seriously unlikely to be so much more efficient at 2500rpm, compared to 1800 that it MORE than compensates for the extra drag at 85, rather than 60mph.
What is more likely is that often when
TransAms (Score:3, Interesting)
I was going to post almost the same information. I was surprised another car also receives the best fuel economy at 85mph; most cars seem to like less than 60mph. Then I found your post mentioning you have a '00 TransAm WS6. My numbers are from a '99 TransAm and an '02 TransAm WS6; both 6-speed manuals. (I upgraded because they were being discontinued.) Like yours, 85mph is best; over 90mph starts eating fuel, and under 80mph loses at least 2mpg. My WS6 has never beaten 24mpg. The '99 reached 26mpg g
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> At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg.
It's comments like this which us Europeans wonder if there's any point in us trying to be green, when Americans are still driving cars which only do 24mpg. For every one of us in Europe that buys a car which will do 50 or 60mpg, there's always going to be some American buying an tank which only achieves the low twenties.
Why do I even bother?!
(yes, I realise European gallons versus US gallons are differnet and I have taken this into accou
Re:Fuel economy (Score:5, Insightful)
I've gotten lazy with this. When I know a stop is coming up, I put the car in neutral and let it coast. After a while, my clutch leg starts to hurt, so it's easier this way. People may think I'm weird, but I do it up to a mile away. The car coasts really well, so I'm usually not going any slower than I should be anyways. :)
Re:Fuel economy (Score:5, Interesting)
I would always just leave the clutch in rather than put it in neutral in case you have to get out of trouble.
I remember once, on the M27 in England, a friend and I were driving to Portsmouth. We'd be put just over a pounds worth of petrol in. Gauge didn't even register empty. It was below that. There's a long downhill bit heading down into the city. He put the clutch in and we coasted several miles at least. (This was in a really old Saab.) Little disconcerting, especially when venturing onto a Motorway in the knowledge that your car may break down is an offense.
Re:Fuel economy (Score:5, Informative)
Re:Fuel economy (Score:5, Funny)
>not to mention your spigot bearing.
Is that something a doctor would sort out, or a mechanic?
Re:Fuel economy (Score:5, Informative)
Re:Fuel economy (Score:5, Informative)
However, the throwout bearing that puts (or rather relieves) 2000 lbs of pressure is more likely to be the issue
But I agree with your main argument, Stick it in neutral and let the clutch out.
Re:Fuel economy (Score:5, Funny)
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which flips the man into the pan...the trap is set...
Re:The Jesus Bearing (Score:5, Funny)
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And if you don't believe him, take it from me. I wore out my spigot bearing -- which is more commonly called the "throw-out bearing" -- doing just that.
Here's some more information about how a clutch works [edmunds.com]. The article has some good information, in particular, about how the clutch, pressure plate and throw-out bearing work together.
So don't take the mechanics word for it. Read it for yourself. ;) This is called 'riding the clutch' and it's considered bad.
Re:Fuel economy (Score:4, Informative)
Re:Fuel economy (Score:5, Funny)
isn't this discussion getting a tad too manly for slashdot?
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As an ex-mechanic, I wouldn't recommend coasting all the time with your clutch in, you're not doing it any favours. Stick the thing in neutral, it's far better for the longevity of your clutch, not to mention your spigot bearing.
You should be doing neither. You should be coasting in gear. You'll gain fuel efficiency while saving your clutch. Modern cars shut off fuel injections above certain RPMs when coasting. Putting it in neutral burns fuel to keep the engine turning.
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Which, do you imagine, is cheaper to replace if you break it? Brakes, or engine?
As long as you don't over-revv the engine, using it to brake shouldn't have any significant effect on its lifetime. No one's saying that you should shift into first gear when you're going 65 mph.
And your brakes not working when you need them can be even more expensive.
Re:Fuel economy (Score:5, Interesting)
Coasting may be illegal in most US states, but so is travelling five miles an hour over the speed limit. Fortunately, most cops aren't about to pull me over for either (though I do stick to the limit in school zones, for obvious reasons), so I'm not liable to not coast simply because it's illegal.
My reason for not coasting? From what I understand, when the engine's turning above ~1,000 RPM, the throttle's at "idle" (no pressure on the pedal), and the transmission's in gear, then the fuel injectors shut off. For everyone but the parent poster, that means it's not burning gas, and thus raising the mileage. Whenever I might use the coasting technique, it's probably better to simply leave it in gear, let the injectors shut off, save gas, and save my brakes (without worrying about overheating them, too).
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
Re:Fuel economy (Score:5, Interesting)
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
This is common on a lot of cars. I drive a 2006 Impreza STI and I can absolutely confirm that the injectors are off if you are engine braking. It's the only time EGT drops all the way to zero, even at idle EGT is still around 700*. My 2003 Mustang Cobra did exactly the same thing with the stock tune, verified in a similar way. I'm pretty sure most cars do this -- at least the manual transmission ones. If you leave the injectors firing at all when engine breaking, it causes popping & gurgling afterfires that sound like an old pickup truck.
Injecctor shutoff (Score:5, Informative)
I have an 2001 Sentra and just inst hooked up a gadget I got from Think Geek (ScanTool, I believe its called) that reads the engine computer through the OBDC2 connector. I can verify that taking my foot off the gas does shut off the injectors if the car is in gear and going fast enough. From the ScanTool manual I infer that this behavior is common, but not universal among cars.
Re:Fuel economy (Score:4, Insightful)
Why the fuck is this modded as redundant? While it may be a case of "Yes, it's against the law, but..." it IS a safety issue. On several occasions I've had to get the hell out of the way quickly, and had I been coasting in neutral, I'd most likely be dead now.
I'd rather have a dead clutch than a dead me. Besides, with a knackered clutch, going up hills is MUCH more entertaining. Will you make it to the top? YOU JUST DON'T KNOW! It's like an extreme sport. Make it to the top of a particularly steep hill with a slipping clutch and you should really go buy a lottery ticket, as you're clearly one lucky SOB.
Re:Fuel economy (Score:4, Informative)
Absolutely. Only thing I used to do to save fuel was slipstream (or "draft" to you uneducated NASCAR fans) trucks. Worked surprisingly well. And of course you can turn off your AC, your heat etc... No ventilation. Keep your windows closed etc... Seriously, there was a Top Gear segment where Jeremy Clarkson drove to Scotland and back on a tank of gas. 800 miles. He details everything you can do to cut down on fuel consumption. You can use your radio and that's about it. Also you need to plan ahead and consider what will happen ahead. Will the car in front slow down? Will you need to overtake etc...
Really, there are SO many things you can do to help fuel consumption.
Re: (Score:3, Informative)
Never post when tired. It was a tank of DIESEL he used. Not gas. (Before some smart arse goes karma whoring trying to correct me.)
Re: (Score:3, Insightful)
Yes, but if you KNOWINGLY enter onto the Motorway with a condition on the car you KNOW could mean you break down, so deliberately driving on with low fuel, THAT is against the law. Regular car problems are unavoidable.
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I mean...I know gas is high, but, if you have to go these extremes...look for a career change, eh?
Re:Fuel economy (Score:5, Funny)
Actually, most cars nowadays with manual transmissions will actually no longer consume fuel.
Wow! That's wierd. There must be something wrong with my 5-speed Ranger then, because it's constantly consuming fuel. About 24 MPG worth in combined city+hightway.
Re: (Score:3, Funny)
Which is why the real hardcore people not only go into neutral on downhills, they also shut their engines off.
Re:Fuel economy (Score:4, Informative)
Some cars have steering wheels that lock when the key is turned off. Recommending that to folks who don't know theirs locks can be a very dangerous thing.
Re:Fuel economy (Score:5, Insightful)
Three reasons this is a bad idea. One, if you suddenly have to swerve to avoid something, you might miss the edge that the power steering would provide Two, you might need to suddenly apply engine power in some emergency situations, like if you hit black ice and start skidding. Three, if you accidentally turn the key too far back, you'll engage the steering wheel lock and won't be able to steer at all.
In short: really bad idea.
Re:Fuel economy (Score:5, Insightful)
Four, if you need to stop suddenly, I hope you have strong legs. Those brakes don't work very well when your system isn't pressurized anymore.
Five, your cooling system may no longer be working, and if you are riding the edge of an overheat (considering you're driving far enough to consider killing the engine on coast) this may push you over the edge.
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Re:Fuel economy (Score:5, Insightful)
Really? How many more miles does he expect to get on that engine before the repeated stress of starting it burns out the starter motor, at a minimum? Seriously, when the engine isn't running, the oil pump isn't running. When the oil pump isn't running, the oil runs down into the drain pan (especially when it's already hot). When you start a car, the cylinders are underlubricated until the oil pump gets things moving again. How many gallons of gas to you have to save to pay for a new engine, both in economic and ecological terms?
Also, keep the jerk in mind the next time you have a smoggy day. What did you think happened to all that unburned gas in the exhaust? Catalytic converters aren't magic, you know.
Re: (Score:3, Insightful)
Not just weird; dangerous for reasons other posters have cited.
Hypermilers are starting to get really annoying. I was behind a person the other day who refused to go more than 30 in a 40mph zone on a road where one cannot pass. The line of cars full of unhappy people behind me got to be very long just in the short 2 mile trip to Home Depot. If that were not bad enough, she took a good half mile just to get to 30.
Driving with an eye towards fuel economy is all well and good. I pay attention to my realtim
Re:Fuel economy (Score:5, Informative)
Engine braking is a fine way to brake, nowadays. Modern engines don't burn any fuel while engine braking, and the braking is usually slow enough for the unlit brake lights not to be a problem.
Plus, it's essential while going downhill, otherwise your brakes will be useless. Unless what you want to accelerate, of course. Then, by all means, let gravity help you.
Re:Fuel economy (Score:5, Informative)
He's not referring to engine breaking down a hill or simply allowing the engine to slow down the car in whatever gear the car is in. He's referring to those that will downshift, engage, downshift, engage all the way to a light.
Re:Fuel economy (Score:4, Informative)
It IS the correct way to drive, but you don't have to do ALL your gears.
I'm a truck driver and my exhaust brake (that will only work in gear) accounts for 60-80% of my stopping power and is a must for hill descents.
http://en.wikipedia.org/wiki/Exhaust_brake [wikipedia.org] the exhaust brake is intended to assist with engine braking, which is something diesels aren't normally good at.
In a car you can get away with anything, but just because you get away with it doesn't make it the right way to drive. (I'll admit I do coast in cars from time to time)
Re:Fuel economy (Score:5, Insightful)
Foolish. All engines are designed to brake.
Brakes are a safety device; you need them for keeping the car stationary, bringing it to a stop from low speeds, to prevent emergency situations from occurring, and for use in an emergency situation itself.
You save them; you don't use them. If you use your brakes as a matter of course, to control your speed, then you won't have them when you need them, due to heat build-up. No, disc brakes won't save you - they're better than drum, but they still heat up and loose effectiveness. No, ABS won't save you, either. It still has nothing to play with if your brakes have heated up.
Which would you rather: a bit of fuel economy (dubious anyway), or a large funeral bill?
Re:Fuel economy (Score:5, Funny)
Btw it wouldn't be my funeral bill would it!?!
Re:Fuel economy (Score:5, Insightful)
Exactly.
If you drive well, you don't use much the breaks. In fact, the ideal ride is one you don't use the breaks at all, save for full stops.
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Re:Fuel economy (Score:5, Informative)
You obviously don't live in the mountains. Here not using the engine for braking is a sure recipe to roast your brakes and have a pedal response like pudding after 10 mls, and yes, we here have slopes of 20 or 30 mls. Coming down from Timmelsjoch to Haiming we start out at ~7500 ft and after nearly 6000 ft we reach Haiming at an altitude of 1800 ft. If you ever plan to do that with only the brakes please send me a message upfront, so I can avoid driving there that day.
I am using my engine for braking all the time, it has now 80,000 mls, no sign of wear and tear, and I have replaced the brake discs once.
Re: (Score:3, Informative)
Sique,
You're right on the money there. I was just driving through the mountains last week (Northern California, Oregon, and Washington). There are plenty of signs to remind truckers to downshift, but nothing for the folks in 4 wheel cars. I've been all over, and know how to drive. It was kinda funny seeing people zip past me and stand on their brakes for a mile downhill. Well, probably more funny in the morbid sense. I was driving an automatic with the speedlimit at 60.
Re:Fuel economy (Score:5, Informative)
Engine braking is not like brake braking, genius. You will not wear out the engine in any fashion doing engine braking in any gear. Unless you plan on shifting into first and engine braking while you're going 65+, in which case you will be driving over your transmission as fast as you shift into gear. Engine braking is simply what will happen if you're not giving your car gas, as a simple answer. The final result of engine braking is that your engine will starve for gas and stall, if you engine brake at a low enough speed.
Diesels specifically do it for longevity and safety when going down steep hills, as well.
how did this guy get modded insightful? It was incorrect information.
Re: (Score:3, Interesting)
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How exactly is it putting "extra stress" on it? You're taking in, compressing, and exhausting air, as opposed to taking in and compressing air, shooting in some gas, blowing the lot up, then exhausting the remains.
Then again, IANAMechanic.
Re:Fuel economy (Score:5, Informative)
The key is to drive a manual transmission and to hold in the clutch whenever you can(especially downhill) so that the car coasts(runs at idle) as much as possible.
Wrong.
If you are at 0% throttle and over a certain RPM most ECUs will turn the injectors off (0 fuel usage). I am sure you will agree that (fuel used to idle the the motor) > (no fuel used).
Should I even bite on your username or just let that one slide?
Re:Fuel economy (Score:4, Informative)
No .. he is correct.
There is a TPS ( Throttle position sensor )
If you have the throttle at idle , and the engine is spinning ( as during a engine breaking ) most modern cars will not open the injector at all. There is NO reason to send fule into the cyl because you are not requesting any additional power from the engine.
Your thinking of a 1967 chevy with a carb you fucking idiot .
Re: (Score:3, Informative)
The engine is consuming no gas when engine braking, as the system is running in reverse. Rather than power going from the engine to the wheels, you've got power going from the wheels to the engine.
Your idea is correct for carburetor, but is completely wrong for fuel injection engines.
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The key is to drive a manual transmission and to hold in the clutch whenever you can(especially downhill) so that the car coasts(runs at idle) as much as possible.
Doesn't help with fuel economy, and will very *very* quickly destroy your clutch release bearing. If you *must* do this, put it in neutral. The 5p worth of petrol you save probably won't offset the cost of pulling out the engine and gearbox to replace the clutch...
And don't engine brake because that is poor form and is retarded. Use your damn b
Re:Fuel economy (Score:5, Insightful)
Holding in your clutch a lot will stuff your throw-out race. That's really bad, because that'll lead to a clutch that you can't engage or disengage. Not having a clutch when you need it is really really bad.
Engine braking is good practice. Putting aside the cleverness of modern ECUs, most footbrakes fade with use as they heat up. Yes, even disc brakes; they're a heap better than drum, but they can still overheat very rapidly.
If you're coming down a very long, steep hill and you're not engine braking, your brakes will be much less effective by the time you reach the bottom.
Brakes are a safety device, not a speed control tool. You brake only when required. Your accelerator pedal and engine braking is what you use to control your vehicle's speed. If you need to use your brakes other than to come to a stop, you're pushing your car too damned hard, or you're too damned close to the guy in front of you. Back off.
Re:Fuel economy (Score:4, Informative)
Please, please, please use your brakes when coming to a stop and NOT your engine.
What if I don't want to come to a stop and just want to stop accelerating when going downhill?
Brakes are balanced to work at every tyre whereas engine braking has the potential to make only half your vehicles tyres rotate at a different speed to the road surface which could cause a spin.
Err ... yikes. If that happens, your vehicle wasn't roadworthy to begin with, or the road conditions were so bad that applying the brakes wouldn't have been that much better.
Also, what you're basically saying here is that accelerating could cause a spin (since it also changes the vehicles speed by using the engine).
Look up any advanced driver training material if you need to know more.
Got any concrete citations for that?
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Wrong.
In most modern manual transmission cars, the ECU will cut fuel to the engine if the wheels are keeping the engine cranking. This is generally referred to as DFCO, or "Deceleration Fuel Cutoff".
Many cars manufactured since 1999 have this feature, according to a quick google search.
Putting it in neutral or holding the clutch down will actually use more fuel going downhill. Not much, but still more than none, and you will accelerate unless you use the brakes, since the engine compression is no longer lim
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Pads are not the problem when riding the brake downhill, brake fluid is. If your brake fluid boils, the brakes are useless. Using up your brake pads just reduces you to metal-on-metal braking (which still works, but it's unpleasant to listen to and requires new/resurfaced discs).
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You're proud of 30mpg?
I can commute about 40 miles to work in the rush hour traffic and get over 50mpg without even trying.
Outside of rush hour, a leisurely cruise back along the same route easily gets 60mpg. Best I've ever achieved was 77mpg, but that was actually quite a lot of work. It's not constant speed that you want for maximum mpg, and although using a cruise control gives a reasonable zeroth order approximation, you can do better. Of course, if the road is 200 miles long, with no variation in incli
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I don't own a car. (Score:3, Funny)
But I've noticed I get less tired if I walk rather than run :-)
My plan for improving the world's fuel consumption (Score:5, Insightful)
Make those fuel consumption displays mandatory.
Most cars these days know their consumption - it's one of the first things they look at when they connect the laptop to the engine when you go for a service.
Make the display mandatory, make it large, and put it in a prominent place. It'll do wonders for everybody's fuel consumption.
Re:My plan for improving the world's fuel consumpt (Score:5, Insightful)
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Tachometers show us speed
Does your car only have one gear?
BMW on fuel efficient driving (Score:5, Informative)
An American Road & Track issue from many years ago (and I'm damned if I can recall which one) had a long article on the results of some fuel economy studies conducted by BMW.
The findings seemed to show that driving style was more important than overall speed.
The tips, in general, were:
- Keep your speed constant; fluctuations up and down are bad.
- Accelerate to your target speed quickly. Spending time slowly accelerating up to it wastes fuel.
- Be in the highest gear feasible for your engine type and road speed.
- 75% throttle for acceleration, conditions permitting.
- Keep your revs low, and change gears often to keep them low. That said, know your torque curve, and use it; if you have a small 4 cylinder, trying to accelerate at 1000 revs is futile.
Re:BMW on fuel efficient driving (Score:4, Informative)
Myth #1
Accelerate to your speed quickly. This actually wastes gas. It's usually touted by people that really dont know how cars work.
accelerate in your engine's economy band. this can easily be found by watching your MPG gauge or using a $12.95 Vaccuum gauge attached to your car's vac system.
Flooring it to your speed wastes gas, you are running rich the entire time putting fuel out your tailpipe. Going to slow wastes fuel as well, accelerate as to what your car's max economy is for that driving situation. problem is most cars are not equipped with the gauges needed to do this. American cars are designed for really stupid drivers, so they remove most of the gauges. too many gauges confuse american drivers.
75% is inaccurate for most cars. If I was driving a high performance car, 75% throttle is burning tires. In a smart car it's too little as it's power band from a stop is a gradual increase from 45% to 80% as your speed increases so you can keep the engine in it's power band for max economy.
Basically you have to learn your car. It takes time and efffort to maximize fuel economy. as well as getting rid of retarded driving habits like the morning dragracing from light to light. Accelerate slowly and time the lights to you never have to stop saves more gas than anything else.
Oh, Car and Driver reviewed those BMW tips, they found that they contradict each other.
Re:BMW on fuel efficient driving (Score:5, Insightful)
Sounds like you really don't know how cars work, then. Older mechanical fuel injection systems or carburetors CAN get better mileage with full-throttle acceleration (if you keep the RPM down using a manual transmission). The reason is the open throttle lets the engine breathe easier so it's not wasting energy drawing air past a restricted opening. BMW and others have experimented with eliminating the restrictive butterfly to improve economy, and of course one of the reasons diesels enjoy better economy is because they have no throttle butterfly.
So, yes, you can improve economy by keeping that throttle open and the RPM low -- as long as your computerized fuel injected engine doesn't perform WOT enrichment (or you disable that feature).
Re:BMW on fuel efficient driving (Score:4, Insightful)
So, yes, you can improve economy by keeping that throttle open and the RPM low -- as long as your computerized fuel injected engine doesn't perform WOT enrichment (or you disable that feature).
Since your "exception" is basically 90%+ of cars on the road, you are actually agreeing with the gp?
Re:BMW on fuel efficient driving (Score:5, Insightful)
tylernt covers the "full throttle" component of this in another reply to your post.
I'm an Australian, and the son of an engineer who restores old vehicles for a living. We have British, Italian and Japanese vehicles. We have never owned an American vehicle.
The BMW data was almost certainly collected for their vehicles, which almost uniformly (at the time of the study) used straight 4 or straight 6 engines. 75% throttle would have been a rough figure arrived at for their own machines, I would imagine.
It is patently obvious that applying more throttle increases the amount of fuel used per second. However, the amount of fuel used is not a direct 1:1 to your acceleration.
The trick here is not that you use less fuel to reach your desired speed by accelerating harder. That's nonsense, and an incorrect understanding of the problem. Accelerating harder may well use more fuel to reach your desired speed. The trick is in how much time your spend at you desired speed, not accelerating.
If you do the calculus on this, you'll note that with the rapid acceleration model, you spend a far greater time at your desired target speed over the course of your journey. While at that speed, you are not accelerating. You will use more fuel accelerating quickly than accelerating slowly, per unit of time. However, your overall time spent not accelerating but simply maintaining speed more than compensates.
Take advantage of aerodynamics (Score:5, Funny)
Thanks! (Score:5, Funny)
I always thought those people were assholes, and I'd fly into a rant about how dangerous and reckless that behavior was. But they're just trying to save money. People really are basically good after all!
Re:Take advantage of aerodynamics (Score:5, Funny)
I've found this is the most effective way to piss off the jackass in the Prius going 65 in the fast lane. Start drafting 'em. They get out of the way quick when they realize I am sucking their fuel efficiency over to my ride, like my car is some kind of mechanized vampire.
Re:Take advantage of aerodynamics (Score:5, Informative)
You actually don't suck efficiency from the car in front of you. That car actually gets a slight boost in efficiency because a second car following close reduces drag-inducing turbulence off the back of the lead car.
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Actually drafting is beneficial to both the lead and tail cars. The lead car gets a boost due to the tail car filling in the vacuum caused by their car. I've never heard if this is actually a measurable difference with non-race cars at highway speeds, but it certainly isn't a good idea even if it is.
Lower speed = Better MPG for me (Score:4, Insightful)
There are sweet spots for driving which is usually specific to the type of vehicle, the gearing, etc. so, to an extent, I'm sure the faster you go the better MPG you will see. But for my car, Mitsubishi Spyder, they recommend shifting into 6th at about 50mph. So basically my interstate driving is all in the top gear by far. At 70-75mph driving on WV interstate highways I get about 20-21 MPG. If I just drop my speed to 65mph everywhere I go during a tank of gas I can reach 24 MPG. I've consistently seen those results out of at least the last 3 or 4 tanks of gas over the last couple months. If I take a US Route (speed limit 55) for 90 minutes to visit my parents my MPG goes up even more for that period of time because I'm going even slower than my usual 65-75 mph. I don't drive too much slower than the posted speed limit (5mph as I state above) because I don't want to feel like I'm crawling but just dropping 5 mph makes a noticeable difference in the range I can achieve with my tank (17.7 gallons). YMMV.
It's about acceleration (Score:5, Interesting)
Other helpful practices: smart braking (Score:5, Informative)
For example, if I see a red light coming up, I'll often ease off the gas and coast in rather than maintaining speed and then braking near the light like most people do. In addition to saving gas on the way to the light, if the light turns green before you stop then you've also saved the gas it would have taken to accelerate back up to speed.
This tactic can be quite entertaining if, for example, an impatient bozo in a SUV comes up behind you while you're coasting, honks, pulls around you and speeds ahead only to stop at the light, and then you smoke him as you coast through the light just as it turns green.
Mod parent up. (Score:4, Insightful)
Re: (Score:3, Informative)
The power needed to overcome air resistance does indeed scale with speed cubed. But you're also going faster. So the total energy losses per distance to air resistance scale with speed squared.
Re: (Score:3, Informative)
The power increases with the velocity cubed, but since a faster vehicle covers the same distance in less time the actual energy used per mile only increases with the square of the velocity.
Acceleration, not speed (Score:5, Insightful)
In a small, aerodynamic car, speed doesn't matter that much. (In a larger vehicle and especially trucks, with their poor aerodynamics, speeds above 60 do start to affect mileage more strongly.)
But how vigorously you accelerate can make a big difference. In the worst of the gas price spike I made a point of accelerating gently and shifting much earlier than usual, and found my mileage improved by 15%.
jack rabbit starts (Score:5, Funny)
I'd say the way people blast off from the green light like their in a Formula 1 Grand Prix* is probably doing a bigger number on fuel economy in city driving more than anything else.
* or not if you were Hamilton yesterday.
more time stuck in traffic (Score:5, Insightful)
This is just another case where people don't realize (or care) that trying to maximize the performance of one part of the system (their commute) ends up diminishing the performance of the overall system.
Only a few people doing this slow driving will result in large numbers of other driver stuck waiting at more lights. Even worse, this kind of slow driving will result in some other drivers driving recklessly trying to get around the slow drivers. It won't take many crashes, injuries, and deaths to completely wipe out any savings made to the economy by a few people driving slowly (if only from traffic backups due to crashes).
Using these kinds of hypermiling techniques are just fine for an individual who doesn't have any regard for how their behavior impacts others.
Re:more time stuck in traffic (Score:4, Insightful)
Your comments implying the driving slower may be more dangerous is laughable - like the tales told of people who got into accidents while trying to buckle their seatbelt.
As the average speed of the US driver has climbed, the death toll has risen as well - both in absolute numbers and in average deaths per mile travelled. There is no evidence that driving slower is more dangerous, notwithstanding your own personal feelings in the matter. And if somebody driving slow in front of you is enough to make you drive in a risky manner, you really shouldn't be driving, should you?
You sure about that? (Score:5, Informative)
The logic is that the majority of people are going to drive at a certain speed on any given road regardless (the "85th percentile" rule) and the one doofus going significantly slower than this becomes a very unexpected, slow-moving obstacle which requires people to either hit the anchors suddenly, or attempt to swerve around, both of which are clearly unsafe behaviors.
While most cops won't care about this excuse because they want to maintain a ticket quota, many judges will, assuming no other violation and a good attitude, accept the "I was just keeping up with traffic" line as grounds for dismissal or reduction of a citation. There's a reason for this.
I grant you that this study, and some others like it, mention only accidents and do not discuss or even mention fatalities, but the reduction of total accidents when everyone drives at the 85th percentile is a pretty clear fact. If everyone drove slower this probably wouldn't be the case, but since we aren't going to change the rset of humanity's driving patterns, telling people to drive slower than they should is dubious advice.
Mod parent wrong (Score:4, Insightful)
Hypermiling isn't even remotely about slow driving. It is about accelerating at an optimal rate, cruising at an optimal rate, and carrying no more speed than necessary to get to the next known stop.
Pay special attention to that last one. Carrying no more speed than necessary to get to the next known stop. A hypermiler's behaviour isn't going to affect anyone. If they were all going to be stuck at the next red light, they were all going to be stuck at the next red light. If they were going to make the light, everyone can cruise at their optimal rate.
A hypermiler's behaviour only impacts how other drivers _think_ they are doing in terms of making good time to their destination. Such other drivers love to do things like see that a light is turning red and then _accelerate_ towards it because they want to be first in line. Or because it just feels good. Or whatever. But they'll be waiting at that exact same red light as everyone else, including the hypermilers.
Posts like yours place the blame here on the hypermilers, but the blame should reside elsewhere.
Re:Mod parent wrong (Score:4, Interesting)
Posts like yours place the blame here on the hypermilers, but the blame should reside elsewhere.
Traffic is a system. I'm not blaming just the hypermilers. I'm blaming anyone who intentionally strives to optimize their own trip while not giving a damn for how that impacts the system as a whole.
I've written and used simulations on traffic and it's pretty easy to demonstrate that one slow driver (one who's slow to accelerate, drives below the speed limit, and/or decelerates slowly) at the head of a pack of traffic will impede the flow of traffic for the entire pack causing the cars behind be stopped behind more lights and spend more time waiting at lights. That one driver may experience better gas mileage but it's at the cost of all the other drivers.
That doesn't even take into account the psychological aspects where the behavior of the slow lead car can result in greater irritation of the drivers behind him and probably erratic driving on the part of one or more of them. If you're going to say that slow driver bears no responsibility in this, then you must also accept that the guy who races up the right hand side and merges late, causing a pile-up behind him also bears no responsibility for the crashes and carnage behind him - for clearly it was the other drivers who didn't respond properly. /sarcasm
A hypermiler's behaviour only impacts how other drivers _think_ they are doing in terms of making good time to their destination.
This is not correct. Let's assume in a case it takes the slow guy 20 seconds to get "up to speed" once a light turns green and the average driver 10 seconds to get up to speed. That slow driver has "eaten up" 10 seconds of the next green light. Had he not been in the way, 10 more seconds of cars could have made it through the next light before it turned red. That means 10 seconds of cars now idling at one more additional light.
Re:more time stuck in traffic (Score:5, Interesting)
You have the wrong feeling. Driving the speed limit is a good idea. Driving far below the speed limit (as the submitter specified people going 30 MPH in a 45 MPH zone) is risky and detrimental to good traffic flow and traffic safety as a whole.
The driver going far below the speed limit is likely to incite someone who is less patient to recklessly try to pass them, and that slow driver is responsible for helping to create that situation. To say otherwise is to tell someone poking a bee hive with a stick is not responsible for the stings they receive - that it's only the bees that should be blamed.
In my eyes, anyone who intentionally drives in ways that are counter to how a traffic system has been designed and implemented is putting the rest of us at risk... that means excessive speeding as well as excessive slow driving.
I've studied traffic a lot over the years and what I do know is that it only takes a few drivers driving in selfish ways to really screw things for everyone else. The guy racing ahead on the right to merge at the last minute tends to be the same guy bitching when someone else cuts them off.
And to say "I'm getting better gas mileage, everyone and everything else be damned" is just more of the kind of thinking that leads to all of us getting screwed.
Don't be aggressive (Score:3, Interesting)
The best hypermiling technique I've found that anyone can do is don't be aggressive on the road. This is pretty obvious but I used to drive like a jerk and weave in and out of cars, constant slamming on breaks and jamming the accelerator. Then gas hit $2.50 and I had a baby on the way so I dramatically changed my driving habits. I coast a ton and never tailgate (well, I do draft behind semis sometimes on the highway). My MPG has gone up a ton and I was basically paying the same at $2.00 and $3.00/gallon for a tank of gas. I do mostly city driving so it's tougher to keep a constant 55 MPH (seems to be my optimum speed), but I just don't drag race from light to light anymore.
The leanset way to drive... (Score:5, Funny)
..is accelerating relatively fast to something like 70mph,
then pull of the engine and roll with no gear until You reach
something like 10mph when You start the engine and repeat.
This is the empirically show best method.
But it will probably irritate other drivers...
Highway versus city (Score:3, Insightful)
But 45 miles per hour does not imply highway driving. It implies driving where the car must stop every mile or so. In this case the energy distribution is different, the dominant term probably being the energy needed to accelerate the car to cruising velocity, which, at 40 miles per hour, with 1 mile stops, occurs perhaps every two minutes. The energy of a car moving with a mass of 'm' moving at 'v' miles per hour is on the order of mv^2. This means that accelerating a car to 45 miles per hour will require twice as much energy as a car that is kept under 35 miles per hour. Now if one is talking about a small car traveling less than 25, and big hemi traveling at 45, then we are talking 4 times as much energy to accelerate the car every few minutes. Of course with a hybrid car some this energy is recovered, but then the rate of acceleration is factor. The faster one accelerates, the less adiabatic the operation, and the less energy is recovered.
So to summerize. In the city, a hemi truck accelerating to 45 miles per hour requires maybe four times as much energy as compact traveling at 30 miles per hour. This energy directly relates to fuel consumption. On the highway where velocity is constant, the domanant factor is merely the energy to overcome friction, which primarily depends on how the engine is constructed and how the shape of the car interacts with the environment. This will probably be slightly different for every car, and every driving style. Thirty years ago it appeared that cars were built to go 80 miles per hour for maximum efficiency. I think it is criminal to drive a Porche slower than that. At the end of the day, for highway driving, it would probably be best to monitor the tachometer for optimal fuel consumption rather the speed. For city driving, slow accelerations with higher speeds only on longer stretches or road.
Best practices (Score:3, Interesting)
What I've found is there's two sets of best practices, depending on the type of driving.
1. Highway driving, dominated by long periods of cruising. With modern aerodynamics, air resistance isn't usually a problem for passenger cars at posted speed limits (up to 75mph). SUVs and trucks have issues, but if you're interested in fuel economy changing to something else is the single biggest fuel savings you can get. Fuel consumption then's determined by two things: how efficiently your engine's turning fuel into power, and how many RPMs it's making. The first you can find by looking at a graph of your engine's power band (power produced vs. RPM). It's a plateau with a drop-off at either end. You want to stay in the plateau region, if you let the RPMs drop too far or climb too high your engine's burning more fuel than it needs to to generate power to keep you moving. The second's mostly determined by what gear you're in. So you want to maintain the speed that keeps you at the low end of the power band in the highest gear you have available. Any slower than that and you need more throttle (and more fuel burned) to maintain speed, or you have to drop into a lower gear and increase your RPMs (which means burning more fuel).
2. City driving, dominated by acceleration from stops. Speed has a small effect, but the biggest fuel burn you have is accelerating away from a stop light. So adjust your speed to match the interval between lights as closely as possible. If you find lights going green just after you've stopped, slow down a bit. And if you find them going red before you get there, speed up. Going faster may burn more fuel, but starting from a dead stop burns much more so you save by avoiding the stop. And don't lolly-gag on the acceleration. You don't want to peel out, but you want to get up to speed fairly quickly so you spend the least time in lower gears. Remember, the lower the gear the higher the RPMs at a given speed and the more fuel you're burning. Plus, getting up to speed smartly makes it easier to judge the speed you need to maintain to hit the next light while it's green. Spend too much time accelerating and you'll either have to hit a much higher speed or you'll miss the next green, have to stop, and burn all that fuel accelerating again.
Fiat punto stationary in fifth 2.5l/100km (Score:3, Interesting)
I noticed that at sane angular velocity there is a peek in efficiency when the turbo kicks in. However, if you go all the way down and let the engine run stationary in fifth gear you can get to a much higher efficiency. I managed to get twice the specified efficiency. The car will be running around 12.5 m/s then (which is about 25 knots)
What happens is that because of the low drag at that speed, the momentum of the car is enough to keep the engine turning above the fuel injection threshold without help. Then the computer decides to stop fuel injection. The result is that the cylinders fire only once in four roughly. Almost any diesel car should be able to do this, as they put way to heavy engines in them.
It won't surprise you to read, that you shouldn't tell your dealer, nor try this on the highway (they have a lower speed limit too).
Please don't ask for help converting this to nautical miles per pint.
Good reader's digest article about this one (Score:3, Informative)
Re:Its not fuel economy, its self righteousness (Score:5, Insightful)
Not that simple (Score:3, Informative)
There is no question that coasting with engine off uses less fuel (zero) but there are other legitimate questions:
Is it necessary? - in many modern cars the fuel is cut off while engine breaking, so in that case you are using zero or thereabouts fuel anyway. On the other hand any savings on fuel (if any) can be easily offset by the extra wear on the brak
Re: (Score:3, Informative)
Air resistance on reasonably aerodynamic objects is actually more like |v|^1.4, not v^2. Air resistance for objects like bricks (or trucks) is roughly v^2.
Re: (Score:3, Insightful)
I second your remarks. If prices were allowed to rise according to supply and demand, then companies would make sure gas got to NC, thus lowering prices.
Re: (Score:3, Interesting)
And here I thought efficiency was related to the ratio of source temperature to sink temperature, and all this seems to do is reduce source temperature.
The water is instantly flash-heated by the fuel explosion into steam for an instant expansion of 1:800 => making all your combustion engines be a partial Steam Engine by piping some H2O mist in through a vacuum tube port.
It's flash-heate
Re:It most likely varies from car to car (Score:4, Interesting)
When the Honda Element first came out, the automatic got higher mileage than the manual. I don't know if they've changed it or not, I haven't looked recently, but they geared the manual different - although the manual had one more gear, it was geared lower than the highest auto gear. The justification was that people who were going to tow or go light off-road would be using the manuals.
Well, some of the people were sick of it, and using similar parts from Acura (which is just high end Honda, of course), they added a sixth gear... and many people went from less than 25MPG to over 30. Would it really have cost Honda that much more to add a sixth gear?