Wednesday, May 30, 2007


High Gas Prices Are Destroying My Fuel Economy

People think they're saving gas by taking it easy on the throttle, THEY ARE NOT!!

I'm hitting more traffic lights and having to sit through multiple light cycles. I'm getting way less MPG.

More time on the road and more stops = more gas burned.

And engines are most efficient between 3000 and 4000 RPM. The extra fuel spent getting up to speed faster is negligible (you may even use less fuel).

(see also, Save Gas: Avoid Brakes!

and, Accelerate Faster

and, Why Peak Torque

and, The Big Deal About Gas Prices

and, Is Gasoline a Giffen Good

and, Horn Day)

UPDATE: An analogy to clarify:
It's also a lot like riding a bicycle. At high gears and low speed, you might not even be able to push hard enough to get going. You mostly just put a lot of strain on the chain. Instead, you use a lower gear and and pedal less hard at a more comfortable, medium pace. Once you get going faster, you'll notice that you're spinning the pedals faster but there is almost no resistance at all and you're wasting a lot of energy moving your feet so fast. So you upshift; resistance increases slightly, your pedalling slows slightly, and you're a lot more comfortable.

It's so true. People (read: backseat drivers) love to say, "What's the big deal with speeding, so you can get there 30 seconds earlier?" I tell them that they are WRONG. Wrong, wrong, wrong. Even a few seconds can make the difference between making it through the light or having to stop. And if you have to stop, it's a minute or two wasted that you'll never get back. Over the long run, driving only two or three miles faster can actually cut a half hour off your trip.

I am willing to spend a few extra droplets of gasoline if it will save me hours, days - even weeks - at the end of my life.
Hello, I'm very interested in your assertion that accelerating quickly to your final cruising speed is more fuel efficient than slowly accelerating to it.

Like many people, I have been doing my best to save gas by improving my driving habits. I drive a 96 Buick Century and was getting roughly 19 miles to the gallon with about a 50/50 mix of interstate and secondary road driving.

In an attempt to save gas, I have changed my driving habits in three ways: I accelerate slower, I coast (i.e. let off the gas, not go into neutral) whenever possible, and I try as much as possible not to break unnecessarily.

As a result, I've increased my gas mileage to an average of 26.6 miles to the gallon, almost a 30% increase in efficiency. Needless to say, I have been very pleased.

As I'm sure you know, accelerating slowly is widely accepted as one of the keys to increasing your gas mileage which is why I am intrigued by you saying that the opposite is true. Would you provide links to sources that support your claim? I want to test your claim out myself by driving in this style for two weeks, but first I want to make sure there is some basis for it.

I do very much agree with your point that braking is the ultimate enemy of fuel economy. Whenever I see a red light approaching, I immediately let of the gas and coast up to it. Many times I'm still going 5-10 miles per hour when the light changes. The strain on the engine to accelerate from 5 miles an hour to 35 is much less than from 0 to 35. Perhaps this is the source of most of my gains in fuel economy?

While I can understand why you hold your opinion about going faster to avoid stopping at red lights, in my experience, this is not the case. In city traffic, I observe that people who accelerate rapidly tend to brake way more than myself. They accelerate rapidly into each successive red light and are forced to constantly brake whereas I never stop and maintain a more constant speed. I suppose they get where they're going fast than me because they occasionally make it through a red light that I end up hitting, but I've (anecdotally I admit) shown that my technique is getting 30% better gas mileage than normal drivers, so to me, the benefits of missing a few red lights do not make up for their car's overall poor performance. This is also true in traffic on the interstate. People are so excited when traffic in front of them begins to clear that they accelerate as fast as possible... into the car in front of them at which point they violently brake again. My technique is instead to maintain a constant speed and let multiple car lengths of space appear in front of me. If people in front of me stop, it still takes me awhile to catch up to them. In addition, this technique aids the flow of cars switching lanes, which is the largest cause of high volume/rush hour traffic (people trying to get on exit ramps, merge into traffic, cross over multiple lanes to make their exit). When you go as fast as the traffic will allow, there is little or no space for people to change lanes thus people have to slow to a stop and turn on their blinker to get in which slows the people down behind them, who bunch up and don't let other people change lets, etc etc.

Anyways, would love to discuss this with you because we obviously have the same goal in mind but very different ways of going about it. Interested in hearing your response! Please CC reply to

I agree with Whitney in the fact that driving at a constant, lower speed saves gas (specially in the city). I am not sure if "coasting" improves your fuel efficiency, I'll try it.

I have to add, based on my experience, that in order to get the best fuel efficiency, you must drive at the highest speed with the lowest rpm as possible (as far as the low-rpm torque of your engine allows you to). This is why you get better mileage in highways that allow you to drive at higher speeds in 5th (or 6th) gear with the same rpm you would be driving at 20mph in 2nd gear in the city.

The higher the RPM, the more fuel you consume.

From this, I have to say I disagree with your point Aaron, that the engines are most efficient in the 3000-4000 rpm range. Perhaps in that range you are getting the most torque/power out the engine, but in what accounts to fuel efficiency, I would say that the 2000-3000 rpm range is better for saving fuel, and it gives you the sufficient torque for going on under most circumstances (say that to go uphill you better make the rpm go up to keep up the speed).

In the cities (in a road trip the following does not apply), the fast driver gets to his destination almost (seconds of difference) at the same time as the easy-going fellow who sustains a constant speed while driving. (try it out: one day drive fast, and the other calm down, you'll see there is almost no difference).

If you get to pass this stoplight in the yellow/red light change by driving fast: you are going to stop (abruptly in some cases) at the next one, making no difference in fuel saving, nor time.

Live peacefully, save gas, contribute for a better world for the future generations: drive calmly, not violently, and, again, live peacefully.

My best wishes,

whitney, the engine is more efficent at higher RPMs, but moving faster takes more power. You use more fuel per mile when you go faster, and you use slightly more fuel accelerating faster all else equal (you can only go as fast as the cars in front of you).

You won't get better fuel efficiency accelerating faster because of all the other assholes who will keep you in check, but it won't hurt your fuel economy much. There would only be fuel savings if enough people did it to get more cars through traffic lights and prevent stops, that's where the real savings are {you want to decrease the frequency changes in speed, and avoid stops [cars can use 6 times as much fuel starting from a full stop vs rolling (5mph)]}.
[by higher RPMs I mean between 3 and 4 thousand, efficiency drops off fast after 4 thousand (many more like 3600)]
I agree with coasting, it will definitely save you fuel. It's very good, especially in light traffic. It'll save you fuel in heavier traffic, but it's rather selfish.

I'm less sure about it in heavy or high traffic. You might be causing cars behind you to stop of slow down. This should only be a problem in traffic when you may a hit traffic light more than once. Once your in a queue, you want to push though it, but if you can avoid queuing...

Sounds like you have it about right, but I don't know what you mean about 2000-3000 rpm being more fuel efficient. If you are cruising, yeah definitely, but I'm talking about accelerating.

2000-3000 is definitely better for cruising at the same speed vs at 3500. 1000-2000 will get you even better mileage, but at that point you'll be lugging the engine which'll end up costing more than the fuel you save.

I'm talking about getting up to your desired speed then laying off, coasting to avoid stops, etc.

And yeah, 4000 rpm is high. Most cars lose efficiency after 3800 and some, more like 3500 rpm. You really want to be between 3000 and 3500, but I wanted to give wiggle room, people avoid such limits (and you shouldn't get the opportunity to get too close to it in traffic too often, not without having to brake).

If everyone had the same driving habits, wouldn't accelerating slower and coasting more be the superior technique for everyone to follow?

I understand what you mean by me coasting being selfish because it at time interferes with the way other people drive. If there's one thing I've learned from this, it's that if I'm trying to save gas for the sake of the environment/reducing US dependency on oil (which I am, though those goals are admittedly a far second from my goal to save money), then I should take into account the externalities of my driving behavior. However, if everyone were to drive as I do, wouldn't there be a cumulative 30% increase in fuel efficiency across the board? (recognizing that 30% is a totally arbitrary number, results would vary by car)

Like I said, I understand that you will naturally avoid more red lights by accelerating faster, but I used to drive like that and had much worse gas mileage. Of course if everyone accelerated at the same speed, then perhaps I would avoid more red lights, but I don't see how it would ever make up for the large increase in fuel efficiency I have observed from accelerating slower. I agree with Andres' observation that people who make it through a red light at the last minute tend to have to abruptly stop at the next red light because most roads' traffic lights are coordinated with each other.

Well anyways, thanks for making me think and I'm glad there are a lot of people out there who are becoming more aware of their driving habits like myself even if I don't 100% agree with them all.

1st, there's probably no reason you should feel guilty since you aren't likely to be affecting traffic much yourself. What inspired my post was that it seemed like more drivers were driving like you suggest (well not exactly, they still speed and don't coast) and causing me to waste gas (and making my techniques less effective--I usually don't accelerate when I'm in a backup, except to prevent cut in, knowing that cars behind me aren't going to fill the gaps and the cars in front are going to take their sweet time, causing me to have to stop, anyway).

Accelerating slowly will save you fuel, all else equal, but if everyone did it, all else wouldn't be equal. Less drivers would get through all lights. Meaning more stops and more fuel.

Again the vast majority of your fuel saving are coming from coasting and avoiding stops. When you accelerate faster, you are at a higher RPM, but for a much shorter time (you might actually use less fuel, but I'm not so sure about that). If you use more fuel accelerating faster, it's not much at all [you should end up with a lower top speed (or alternatively, spend less time at top speed) and higher average speed].

Accelerating slowly also mean more variation of speed. The means you're more likely to interfere with traffic. It also mean less safety.

And you should treat yellow lights like red light. I certainly didn't mean to suggest people should run them. You're supposed to stop, but you're allow to go through if you can't.

My Save Gas:Avoid Brakes post might make things clearer (or maybe worse). And the book The Goal (doesn't address complex systems like traffic directly) is a very good primer on optimizing systems.

I don't plan on doing it, but you might want to look into it. You can get an engine shut off switch for traffic lights. You can turn the enging off and it automatically restarts when you take your foot off the brake. Wear is supposed to be insignificant and you get a net fuel savings shutting off for any stop longer than 3 seconds.
Higher RPMs means more fuel within a gear (but also more power and speed up to peak torque). But a higher gear can use more fuel at a lower RPM than a lower gear at peak torque.
Simplified description:

The load on the engine is about same regardless of which gear you are in, the RPMs being lower than peak torque doesn't mean much for fuel consumption. A lot more fuel is injected into the cylinders to provide the power needed to keep the car going at the lower engine speed, and power transmission (for acceleration) is less efficient at the lower engine speed.

Think of your car's mechanics as a set of springs that transfer all the forces acting on the car to the crankshaft. In order to provide the same power in less cycles per minute, more power must be provided by each piston in each cycle (more fuel, bigger explosion). But at slower engine speeds, not all of the blast energy is transfered to the crankshaft. The piston doesn't move as much during the blast (the blast occurs in a smaller space) so more of the engergy becomes heat (some of the blast also becomes strain on the piston rod and cylinder). At about 3500 rpm, the pistons are moving at a speed that allows the explosive force to best transfer to the piston (the explosion is pushing as much as possible on the piston during the entire stroke). After 3500 RPM, the piston is moving so fast that not all of the explosive force acts on the piston.

It's also a lot like riding a bicycle. At high gears and low speed, you might not even be able to push hard enough to get going. You mostly just put a lot of strain on the chain. Instead, you use a lower gear and and pedal less hard at a more comfortable, medium pace. Once you get going faster, you'll notice that you're spinning the pedals faster, but there is almost no resistance at all and you're wasting a lot of energy moving your feet so fast. So you upshift, resistance increases slightly, your pedalling slows slightly, and you're a lot more comfortable.
The reason i accelerate more slowly (than I used to when I wasn't too concerned about fuel efficiency) is that I can keep a good distance between myself and the car in front of me (in case they rapidly slow down), and be able to react to traffic further ahead of me. I really don't see how faster acceleration is more efficient on the road (maybe in the lab). In real life, reacting to traffic early and planning ahead are what will save the most gas.
Good for people to know.

I do something similar. I try to do just under what the car ahead does (sometimes even less if I expect them to stop) or go around them if possible. I give myself space and increase the amount of space as I get up to speed, to react and avoid braking. If you're accelerating so much that you end up needing to brake, you end up wasting a lot of fuel. Whenever you brake, you basically have to redo your previous acceleration.
How to most efficiently accelerate is a very complex problem. It involves the vehicle's engine map, torque converter lock up (for automatic transmissions), and the fact that all else being equal (which it isn't) it's more efficient to accelerate slowly since a given amount of kinetic energy is added over a longer distance.

There are other variables as well, but I think these are the main ones. I can't find an engine map for my vehicle and, try as I might, I can't discern the characteristics of the torque converter, so I haven't solved the problem for my vehicle.
Post a Comment

<< Home

This page is powered by Blogger. Isn't yours?