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Fuel Efficiency and Slow Driving?
Posted by
timothy
on Sun Oct 12, 2008 05:57 PM
from the practical-interest-here dept.
from the practical-interest-here dept.
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?
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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.
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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. :)
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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.
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Re:Fuel economy (Score:5, Informative)
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Re:Fuel economy (Score:5, Funny)
>not to mention your spigot bearing.
Is that something a doctor would sort out, or a mechanic?
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Re:Fuel economy (Score:5, Informative)
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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.
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Re:Fuel economy (Score:5, Funny)
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Re:The Jesus Bearing (Score:5, Funny)
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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.
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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.
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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.
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Re:Fuel economy (Score:5, Funny)
isn't this discussion getting a tad too manly for slashdot?
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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.
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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.
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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.
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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.
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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.
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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?
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Re:Fuel economy (Score:5, Funny)
Btw it wouldn't be my funeral bill would it!?!
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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.
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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.
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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?
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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.
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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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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.
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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).
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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.
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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!
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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.
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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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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)
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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?
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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.
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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.
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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.
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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...
Re:Its not fuel economy, its self righteousness (Score:5, Insightful)
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