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.
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.
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.
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)
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:3, 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.
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 brakes.
Makes no difference to fuel economy. I suspect that using the footbrake rather than engine braking will actually make you use *more* fuel. On damn near any European car built in the last 25 years, the engine will use no fuel at all when you're engine braking.
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: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.
scangauge (Score:2, Informative)
Resistance (Score:2, Informative)
Air resistance.
Tyre rolling resistance.
Then approximately in order you have;
Air conditioning off.
Engine RPMs constant and at the peak of the torque curve if you can.
Clean your air filter.
Fit iridium spark plugs.
Use a fuel with a cleaning agent every 6 months or so.
And probably illegal;
Chip your car so it runs at the ideal gas/air mixture, not simply one which will allow it to pass the regulations.
Get rid of your catalytic converter.
Re:Fuel economy (Score:2, Informative)
Driving in neutral or with the clutch engaged is also illegal in many states.
That said, I do do it sometimes when the coast is clear and it looks safe. It's a bit harder now that I usually drive an automatic... have to get the rpms at the right place before putting it back in D ... never got my wife to learn to drive standard).
Also note that engine braking is absolutely necessary in mountainous / hilly terrain. When your brakes overheat from riding them down a long hill, they kinda stop working. Then your wheels catch fire.
But when it's safe to do so, certainly use your brakes rather than your engine. Engine braking doesn't really use fuel, but it's better to wear down your brake pads as opposed to wearing down your engine.
Ideally, you want to try to drive such that you don't use your brakes at all. After all, braking means you're converting your kinetic energy into heat (unless you're lucky enough to drive a hybrid with good regenerative braking). If you never brake, then that means you're never hitting the gas more than you need to, so you're not wasting.
Anyway, if you also make sure your tires are inflated properly, and make sure you always drive in the highest gear you can as much as possible, you should already be in pretty good shape.
Re:What works: (Score:2, Informative)
It's counter-intuitive, but relatively rapid acceleration is far better than slow acceleration.
A long, slow acceleration up to a target speed will use more fuel than a rapid acceleration up to a target speed. This is regardless of the vehicle's horsepower or torque.
Downhill coasting I've covered in other posts in this thread. Suffice it to say that brakes are a safety feature that you don't to have missing in an emergency.
Cruise Control? (Score:2, Informative)
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 brakes. You could also cause major damage to your transmission if you shift into a low gear or even into reverse by accident while moving at high speed.
Is it safe? - No. It's a bad idea to drive in neutral as you cannot quickly accelerate should the need arise, and also increased strain on the brakes makes it more likely that they will fail at the wrong moment.
Is it legal? - Depends where you live, in California and probably some other states it is actually illegal - look up California Vehicle Code 21710
Re:Fuel economy (Score:5, Informative)
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:Fuel economy (Score:3, Informative)
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).
Re:Mod parent up. (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:Fuel economy (Score:5, Informative)
Re:The big cost is braking (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:My plan for improving the world's fuel consumpt (Score:2, Informative)
Oh yes. My car's got one of those displays and my fuel efficiency is about 10 MPG higher than it was when I first got the car, because I pay attention to how my driving affects efficiency.
Re:Mod parent up. (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.
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: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:Fuel economy (Score:0, Informative)
Actually, most cars nowadays with manual transmissions will actually no longer consume fuel. This has largely been the case for about 5 years.
Also, check your regulations. In a lot of states, it's actually a citeable moving violation to be in neutral like that.
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:3, Informative)
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: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: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. I'd brake until I knew it was too long and too much of a grade, and then pop it down into 3rd so the engine could drag me along. I was leaving the truckers plenty of room, but holding their speed downhill so we were all safe. I knew I'd have a chance to pass them on the next uphill section.
Standing on the brakes can boil your brake fluid (which is what creates brake fade). Continuing to do it can bring the brake fluid up to the flash point, which won't be pretty if you get there. 4 wheels on fire, and no way to stop to get out. It doesn't happen much. People usually crash into something first, but it can happen.
There's a big difference between engine braking because of the conditions (like rolling down a mountain), and downshifting hard through all the gears for every single stop light. Brakes work a lot more efficiently.
Last time I was out racing (autocross style on a track, not street racing) the instructor pointed out that I was too reliant on downshifting for my turns. I was slowing down too early. He pointed out "you have antilock brakes, use them". I'd come a lot closer to the turn before slowing down, which put me way ahead time wise, but in the same place on the track at the right speed. The downshifting was only to prepare for coming out of the turn, and a little extra stopping power.
But, back to the topic, normal street driving. It's easier on the car just to throw it in neutral and roll. standing on the clutch leaves the clutch spinning against the disengaged clutch, which wears the clutch a little. Downshifting or braking are just bleeding off speed that you wasted gas to come up to. Rolling at idle to the stop and taking your time saves gas there. I know my car very well, as should every driver, so when I see a light turn green ahead that I was slowing for, I just go into the appropriate gear and start going again.
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:The big cost is braking (Score:2, Informative)
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:3, Informative)
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 limiting wheel speed.
Scooters are part of the answer.. (Score:2, Informative)
The biggest problem I see on the roads are large cars being driven around with only 1 person in them! How "fuel efficient" is that?
About 9 months ago I got sick of paying high prices for petrol and wanted to do my bit for the environment, so I got my motorbike license and purchased a 125cc scooter. I haven't looked back. Now my wife and I only have two vehicles, a small 2.0 litre car and a 125cc scooter. These two vehicles are fine for everything we need them for.
While I admit scooters aren't right for everybody, they are definitely a possibility for the majority of people. When I try and encourage other people to do their bit and buy a scooter they often come back with the following responses...
"They don't work out that much cheaper once you buy everything." - Complete BS, sure there's a little bit involved in the outlay (Scooter AUD$3000, license/training AUD$500, gear AUD$500, rego AUD$350, comp. insurance AUD$200) but once you're up and running these things run on the whiff of an oily rag. Consider it an investment. Right now I'm doing about 150km per week to and from work (and a little bit of running around on the weekends), I'm averaging about 30km per litre! It costs me around AUD$9 per week to fill up, that's with 98RON premium mind you! So you do the sums and see how much it will save you.
"Scooters are dangerous, you could get killed." - Again, complete BS. I was sceptical about how everyone kept saying how dangerous riding a scooter would be. So I did my research and contacted my state's Transport Authority. The stats were even surprising to a sceptic. In the past four years in my state there's been one fatality on a scooter and only three other fatalities on motorcycles less than 250cc. Once you go over 250cc, people seem to become retards and there are a lot more deaths. Mind you, 2/3 of those were with stationary objects i.e. poles, parked cars etc. People forget too that on a scooter you're a much smaller target and much more agile. The main risk I've faced seems to be people coming into my lane without checking blind spots. In all cases I've been able to successfully swerve/break/accelerate out of the way.
"What about riding in the rain, you'll get wet." - No, you don't if you have the right gear. I bought a weatherproof clothing straight up. I wear this gear over my normal business attire with leather shoes and I'm yet to get wet. I've even ridden in hail and I didn't feel a thing!
"You can't carry much stuff on them." - This is the only place where scooters fall down slightly. But still, scooters offer a few storage options. Most come with storage under the seats - big enough for a bag or your helmet. There's usually a bag hook to hang some shopping bags, you can get a top box put on the back for more storage and I also wear a back pack.
"They're too slow and therefore dangerous." - This is true for the 50cc scooters, which I believe are only suitable for inner city riding. But my 125cc will top out at about 100km/h and will happily cruise at about 90km/h. They are also *very* quick off the line.
Did I also mention they're very FUN to ride?
So what's your excuse?
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.
Re:Fuel economy (Score:3, Informative)
Re:Fuel economy (Score:3, Informative)
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.
Re:Take advantage of aerodynamics (Score:3, Informative)
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.
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.
Re:Fuel economy (Score:2, Informative)
Don't worry, I completely agree with you, and I use engine braking all the time by downshifting as I pull up as it prevents wear on the brakes. The car also pulls up "flat" rather than pushing all the weight forward onto the front brakes as that's where most of the weight transfers to when you apply the brakes. The only time you'll do engine damage by downshifting is if you do it too early and watch the tachometer coming back from above the redline.
All my point was that if you want to be idling (either stopped or nearly stopped), then you should be using neutral not riding the clutch. :)
Re:Fuel economy (Score:3, Informative)
tire pressure! (Score:2, Informative)
Astonishing that I haven't seen anything about that in the comments (though I haven't read all of it yet): make sure to have appropriate tire pressure can make a huge difference as well.
Go with at least the suggested pressure, maybe even a little (up to 0.5 bar) above and you will notice a difference in fuel consumption too.
Don't get over 0.5 bar more than the suggested pressure because it may effect the tire grip in a bad way.
This will affect your fuel efficiency no matter if you drive fast or slow.
Re:Fuel economy (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.
more info (Score:1, Informative)
You're right, but the principle applies equally to modern EFI systems. Internal combustion engines are more efficient at full load because power output is increased proportionally more than losses are. (Losses are largely dependent on engine speed rather than the amount of torque being produced.) Combustion efficiency itself also benefits at full load from increased dynamic compression ratio, better mixing of fuel with air, and a decreased surface area to mass ratio (better thermodynamic efficiency).
The test conducted by BMW that I'm familiar with used a 528i as the test vehicle (circa 1980). This is a 2.8l straight-six with EFI. They acclerated the vehicle to a set speed, cruised for a bit and then stopped after a set distance. Four variations were mentioned: (listed from most efficient to least efficient according to BMW's results)
throttle 75% open, up-shifts at 2000RPM
throttle 75% open, up-shifts at 5000RPM
throttle 25% open, up-shifts at 2000RPM
throttle 25% open, up-shifts at 5000RPM
So the recommendation is not to accelerate as quickly as possible, but to keep the engine operating near full load at low RPMs. Note that using 75% throttle rather than 100% is specifically to avoid having the fuel injection switch to a richer mixture as it does at WOT.
At this time BMW was working on the 528e. Since the straight-six runs nice and smooth at low RPMs as it is, they designed one that with a shorter camshaft duration and higher intake air speed for greater low-end torque. They also used taller gearing. IIRC they were able to achieve 40mpg at 37mph in top gear. Although this came at the cost of reduced peak power output. Modern tech like variable valve timing/lift and variable length intakes are aimed at getting the best of both worlds.
saab 93 estate (Score:2, Informative)
We've been testing it's mpg a lot recently. Town driving it gets 35-40mpg depending on conditions and traffic. This at speed averaging around 30mph. On Motorways at 70mph, consumption is about 40-42mpg.. at 60mph we easily get 51-53mpg.
The economy savings between 70 and 60mph are quite considerable. At £1.20 a litre (for diesel) that makes quite a difference on a long journey.
I always believed that cars optimum fuel economy had gotten better at higher speeds, but that's clearly not the case in my Saab.
it might be a Vauxhall/Opel Vectra underneath, but it's still a nice car to drive!
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:Fuel economy (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.)
Real World Test (Score:2, Informative)
Re:Fuel economy (Score:3, Informative)
Re:Fuel economy (Score:3, Informative)
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.
It's not (all in) the speed you drive... (Score:2, Informative)
It's not in the speed you drive, it's in the way you drive. It's in little things like:
- Don't rev the engine above 2500 rpm, change gears faster.
- When a traffic light ahead is red, take your foot from the accelerator. Modern engines use no (that's right: NO) fuel on engine braking.
- Anticipate to the traffic ahead, e.g. give room for somebody so you can slide into his spot.
- Don't start the engine until you are ready to set off.
And this will absolutely NOT make you drive any slower, but I've seen tests where they showed you could save somewhere in the range of 6 to 15% fuel.
Here's a nice link to a Dutch programme called "the new drive":
http://www.hetnieuwerijden.nl/english.html [hetnieuwerijden.nl]
Good reader's digest article about this one (Score:3, Informative)
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?
The Factors Affecting Fuel Efficiency (Score:2, Informative)
In a nutshell, yes, the horsepower requirements increase with added speed as your engine must fight with increased drag. So driving slower mitigates the energy required to overcome the additional force.
HOWEVER, going too slow and you don't cover enough distance for the amount of gas you use. In an extreme example, idling at 0mph is an asymptotic point on the mpg graph. All cars have different "sweet spots" where they are optimal. Those "sweet spots" are typically dictated by particular RPM ranges and the corresponding speeds by which a chosen gear will drive the car within those RPM ranges.
Some people will say that simply driving in the highest gear at the lowest speed will always produce the best mpg. Not necessarily so (though generally so). As an example, a 2006 Jeep Wrangler turning 35" tires gets exactly the same mpg in 6th gear as it does in 5th gear at 75mph. This is counter-intuitive but one must remember there are more factors at play within the engine computer than simply RPMs. Looking deeper at the situation reveals that while 5th gear requires more RPMs, the ignition timing is advanced nearly 20 degrees on 6th gear plus the higher RPMs are actually in the engine's torque zone meaning it requires less air (and hence less gas based on the stoichiometric ratio) to produce the same amount of power.
simple mechanical things (Score:2, Informative)
Engine designers attempt to limit this as much as possible with something called a "windage plate", but, for over-the-road cars, it's a bit of a compromise. There are height constraints on the engine, so the oil pan can be only so large, and drivers don't want to be messing with their oil on a regular basis, so efficiency loses out a little bit when they pick a "top" oil level. Try not filling you oil up to that level. Your car will run fine with the level at the "low" marker on your dipstick and you can reduce windage a bit, which should increase mileage.
You'll need to check your oil level on a regular basis, and, of course, there's no advantage in a dry-sump engine - but I don't think any current autos have those.
While I'm on reducing internal resistance, think about using a lower weight oil, that will keep viscosity at high temps. Both Mobil One and Amsoil are good at this.
The notion here is that another point of resistance in your engine is the oil in your main bearings. These are "plain" bearings, which are actually "oil wave" bearings in that the crank actually rests on a hydrostatic wave. You don't need more viscosity for this than the viscosity of the spec'ed oil at max temp. Better to get an oil that starts out pretty close to this viscosity and then stays there.
Oh - and nobody's mentioned cleaning injectors. Yes, they clog, and they don't all clog at the same rate, so you end up with some cylinders running richer than the others. Not good for max power - or efficiency. Cheap partial fix: start using Techron. Complete fix: take 'em out and send to a specialist shop (note: talk to folks about specialist shop about Techron - all the ones I've spoke with use it). If you're driving a sportscar, there'll be higher flowing injectors available. Interesting for this discussion as you can then pick up a used set of stockers for cheap, send [i]those[/i] to the shop and not be without your ride for long.
Also: has nobody actually mentioned tire pressure, and, for that matter, tire design? there are tradeoffs here, too. Lacking a pyrometer, just try running your tires a bit over spec'ed pressure. There used to be a rule about pressure increase from cold to hot, but that was for bias tires and likely doesn't apply any more. If you've got a pyrometer, you want a nice even temp increase across the treads. And you want [i]dry[/i] air. Nitrogen is best. Scuba-air is second best (it's been very dried out). Otherwise, ask whoever owns the compressor if it's been drained recently. Try a body shop or garage where they're using pneumatic tools ,as moisture's bad for the tools. (time was, body shops were best because of the care they had to take for the paint guns, but they're not run off the same compressors any more).
Look at fuel mileage ratings the next time you're replacing a suit of tires - and remember that you're trading off mileage for something else, but for folks seriously considering some of the driving tactics advocated here, this shouldn't matter much.
Ditto aerodynamics. Air dams are easy to install, do actually work, even sub 65MPH. Get a flexible one so when you hit a curb with it when parking it won't matter.
Oh - and you could just buy a higher mileage car. My wife got one of the first US Priuses. She drives it hard and has been getting 50+ MPG for years now.