Fuel Efficiency and Slow Driving?

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?

Comments

Fuel economy

[JWSmythe]

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

[JohnnyBGod]

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

[MentlFlos]

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

[cailith1970]

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.

Re:Fuel economy

[Sique]

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

[cailith1970]

Definitely a mechanic ;) The spigot bearing is the little bearing in the back of the crankshaft that the input shaft of the gearbox fits into to hold it at both ends. When the clutch is not engaged, the crankshaft and input shaft spin together, so the spigot bearing doesn't spin (well, it spins at the same speed as the crankshaft, if you get what I mean). These little bearings aren't really designed for serious long term spinning, so constantly riding the clutch is going to wear it out pretty quickly.

Re:Fuel economy

[Big Smirk]

In the US they are more commonly called "pilot bearings":

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.

BMW on fuel efficient driving

[base2_celtic]

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

[Cordath]

There are lots of little things you can do to save on gas. Many center around efficient stopping.

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.

You sure about that?

[rantingkitten]

This study commissioned by the US DOT says otherwise, as do thousands of engineers across the country. I personally find this an interesting if dry read, because it's pretty damning evidence that speed limits are set artificially low for revenue generation purposes, since it can be demonstrated that posted limits have a negligable effect on how fast people actually go. Anyway, some things of note:

• Accidents at the 58 experimental sites where speed limits were lowered increased by 5.4 percent.
• Accidents at the 41 experimental sites where speed limits were raised decreased by 6.7 percent.

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:Take advantage of aerodynamics

[tinrobot]

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.