Does engine RPM affect gas mileage?

[ShawnD]

It's commonly believed that running your car at a lower RPM (highest gear possible), will get better gas mileage. We hear this from our parents and friends, and we assume it's true because it seems to make sense, but has anybody ever done a study to prove this?

I'm asking because I monitor my car's gas mileage very closely, and it seems to make absolutely no difference how I drive. The car will get the same mileage regardless of me driving fast or slow, high gear or low gear.

 

[Integral]

Assume that you travel a set distance at the same speed, first in 3rd gear, then in 4th. Can you see that the engine will turn more times in the lower gear. Each revolution of the engine will "consume" the same amount of air/fuel mixture. Therefore you must consume more air/fuel mixture at the lower gear.

Now a lot of modern engines are getting smarter about feeding fuel so, the assumption of a constant a/f mixture may not be valid. With intelligent fuel metering the millage difference may be small.

The biggest difference to fuel consumption is your rate of acceleration. If you like to feel some acceleration and take pride in your ability to get to 60mph (100kph) then you will see improvement by playing "old lady" for a while.

You do more work when doing 5s to 60 vs 10s to 60. This increased work MUST be reflected in fuel consumption.

 

[scorpa]

I think it makes a difference, just calculate the difference in gas mileage a standard transmission vehicle gets when someone who drive one well drives it against someone who sucks at it.

 

[Danger]

There is a reason for low gears, though. 'Lugging' the engine by trying to use a higher gear than is called for is extremely wasteful and not good for the engine.

 

[Ivan Seeking]

Even with intelligent fuel and air management, there will still be an optimum choice for any given speed and load. That is part of the idea behind hybrids - the engine can always be operated at peak efficiency by precisely controlling the generator load and engine RPM.

 

[Stingray]

There are plots that can be looked up for a typical engine's torque output versus rpm and fuel consumption. It is typically better to keept the revs as low as possible, but it is certainly possible for fuel consumption to start getting worse before the engine starts lugging. Besides "direct" efficiency issues, you also have smaller oil and bearing losses at lower rpm.

 

[ShawnD]

Assume that you travel a set distance at the same speed, first in 3rd gear, then in 4th. Can you see that the engine will turn more times in the lower gear. Each revolution of the engine will "consume" the same amount of air/fuel mixture. Therefore you must consume more air/fuel mixture at the lower gear.

That's not true at all. In third gear I can go 50km/h when barely touching the gas pedal. To maintain a speed of 50km/h in fifth gear, I need to floor it. lower rpm * more fuel per ignition = similar amount of fuel being burned.

You do more work when doing 5s to 60 vs 10s to 60. This increased work MUST be reflected in fuel consumption.

That's not true either. Kinetic energy is entirely determined by speed; time is not a factor.

you also have smaller oil and bearing losses at lower rpm.

This seems entirely possible.

 

[nsimmons]

It depends, not on rpm as much as the pulse width of the injectors (assuming efi). low rpm lugging = longer injection time, vs high rpm small injection time.

Car and Driver did a study a few years ago and concluded that moderate acceleration (60-70%) and short shifting (couple thousand below redline) was optimal.

For cruising pick a gear that allows you to keep your foot off the gas. You can track these with a laptop and make nice graphs.

 

[Integral]

That's not true at all. In third gear I can go 50km/h when barely touching the gas pedal. To maintain a speed of 50km/h in fifth gear, I need to floor it. lower rpm * more fuel per ignition = similar amount of fuel being burned.


That's not true either. Kinetic energy is entirely determined by speed; time is not a factor.


This seems entirely possible.

Someone needs to retake (or may be take?) a physics course.

Remember Newtons laws? F=ma?
Now re read my post and make a bit of an effort to understand it.

 

[BobG]

It's commonly believed that running your car at a lower RPM (highest gear possible), will get better gas mileage. We hear this from our parents and friends, and we assume it's true because it seems to make sense, but has anybody ever done a study to prove this?

I'm asking because I monitor my car's gas mileage very closely, and it seems to make absolutely no difference how I drive. The car will get the same mileage regardless of me driving fast or slow, high gear or low gear.

Where you drive makes more of a difference.

If you have to accelerate many times during your trip, you're going to get worse gas mileage. Toss in the fact that the main reason you're having to accelerate a lot is because you're spending a lot of time sitting still at a stop light with the engine running. In the city, being able to time the lights and avoid stops and starts will have more of an effect on your mpg than how fast you actually drive.

If you drive on expressways or long highways, I don't think your speed matters a lot until you hit some high threshold, where upon your mpg stops to drop fairly quickly and that's directly related to the number of rpms the engine is running. That high threshold varies from car to car. I think most of the vehicles I've had can handle up to around 65 mph before they ever seem to start doing any real work, but that depends on the engine's capabilities vs the car's weight and aerodynamics.

As to which gear, you're probably talking about the choice between two different adjacent gears. If you chose to drive around in 1st gear instead of 4th, you'd definitely get worse gas mileage, at least until the car overheated and died - after that, you'd save quite a bit of money in gas.

Edit: I have noticed I get somewhere between an extra 1 and 2 mpg on my first couple of tanks when I'm headed East on vacation vs my mileage returning home. Dropping 4,000 feet in 600 miles is a lot better than gaining 4,000 feet in 600 miles. Plus, the prevailing winds tend to be towards the East and Jeeps have the aerodynamics of a brick.

 

[Integral]

Ok Shawn,

I just returned from a short drive where I obsereved various conditions.

I drive a '94 Ford Probe with a 5 speed manual trany and a 2.5L V6.

@55mph

3rd gear -> 4500RPM
4th gear -> 3500RPM
5th gear -> 2500RPM

going from 3th to 4th I had to let off on the gas, to reduce the RMP, agian to get from 4th to 5th I had to let off on the gas.

Now in 5th gear I am running near the low end of my power band, to accelerate I would drop back to 3rd or 4th and run the RPM up to the new desired speed, then shift back up to 5th to cruise.

Can you see that it must take more gas to travel a mile in 3rd gear at 4500RPM then in 5th gear at 2500RPM?

That is at constant speed, the real gas consumption occurs when you accelerate. The harder your acceleration the more gas you will use. That is simple physics.

 

[Moonbear]

Maybe it's time for Shawn to get a new transmission if he has to floor it in 5th gear to get to 50 km/h when he's barely touching the gas in 3rd, unless they number the gears in reverse order on Canadian cars.

 

[Kurdt]

Depends if he means he's flooring it from a standing start or from 2nd to 3rd or second to 5th. Then you would have to floor it because you'll be way out of the power band going 2nd to 5th. Once you hit 50 you can cruise in 5th.

 

[ShawnD]

Maybe it's time for Shawn to get a new transmission if he has to floor it in 5th gear to get to 50 km/h when he's barely touching the gas in 3rd, unless they number the gears in reverse order on Canadian cars.

In Canada you go through the gears in the order 1-2-3-4-5. Do you start 5-4-3-2-1 in the US? 5th gear at 50km/h is about 1500RPM.

 

[Averagesupernova]

An engine does NOT necessarily take more fuel when turning more revolutions in a given period of time. How far you have your foot into it is also NOT a direct indication of how much fuel you are sucking in a given period of time. RPM and throttle position together will usually determine pretty accurately how much fuel is going in. Engine vacuum is a fairly reliable (relative, not absolute) indicator of how much fuel is going into an engine.

 

[Moonbear]

In Canada you go through the gears in the order 1-2-3-4-5. Do you start 5-4-3-2-1 in the US? 5th gear at 50km/h is about 1500RPM.

No, lowest is 1st gear here too. That's why I was puzzled. If you're flooring it in 5th and not doing any better than that, it sounded like a transmission problem to me. But, kurdt's explanation that you might have meant putting it in 5th when it didn't belong in 5th made some sense as to what might be going on too. Of course, shifting too soon is going to do a lot of weird things both to mileage and the car, but if you're using your gears as you're supposed to, then Integral's explanation is the most sensible.

 

[Ivan Seeking]

Cars are mostly designed to be most efficient in the highest gear while at normal highway speeds, but this characteristic is not intrinsically true of engines. See page three of this pdf.
http://www.oznet.ksu.edu/library/ageng2/L886.PDF

Note that the best fuel economy for a given power requirement does not always occur at the lowest engine speed. For an output of 50% of the maximum horsepower of the engine, the most efficient operating point is about 63% of the maximum RPM - at 102% of the minimum fuel consumption.

The most efficient operating point [fuel consumption/Hp]_min for the engine overall occurs at 58% of max power, and about 69% of max RPM. This would be the desired operating point for a hybrid's engine.

Note also another motivation for hybrids - the terrible inefficiency of the engine as we approach the no load condition. I see this as a huge advantage for hybrid drivers in large cities. In bumper to bumper, stop and go freeway traffic, the fuel savings should be tremendous. So I suspect that some hybrid drivers will get a much greater advantage than might be expected.

[Ironically, from an environmental engineering pov, it would make the most sense to allow standard vehicles to use diamond lanes, and make the hybrid drivers sit in traffic]
Late edits

 

[Integral]

An engine does NOT necessarily take more fuel when turning more revolutions in a given period of time. How far you have your foot into it is also NOT a direct indication of how much fuel you are sucking in a given period of time. RPM and throttle position together will usually determine pretty accurately how much fuel is going in. Engine vacuum is a fairly reliable (relative, not absolute) indicator of how much fuel is going into an engine.

What did you say?
RPMs is not an indicator, nor is throttle position.

Except that throttle position and RPM are an indicator?

Make up your mind!

 

[structural_guy]

Wind resistance is enemy...

It's commonly believed that running your car at a lower RPM (highest gear possible), will get better gas mileage. We hear this from our parents and friends, and we assume it's true because it seems to make sense, but has anybody ever done a study to prove this?

I'm asking because I monitor my car's gas mileage very closely, and it seems to make absolutely no difference how I drive. The car will get the same mileage regardless of me driving fast or slow, high gear or low gear.

Lowering your RPMs and speed will greatly increase efficiency and lower the drag force working against your vehicle. The effect is pretty dramatic in most cars above 60mph.

 

[Averagesupernova]

Integral: What I mean is that without knowing what both throttle position and RPM are doing at the same time you cannot say what fuel economy is doing. In other words, you cannot just simply say (as you have) that more RPM guarantees more fuel consumption no matter what. There are more sides to this than just RPM.

Edit: I said that throttle position is not a DIRECT indication and that an engine does not NECESSARILY take more fuel at higher RPM. You imply that I stated:

RPMs is not an indicator, nor is throttle position.

I would read the above quote as stating that RPM and throttle position should never be considered at all.

 

[russ_watters]

Averagesupernova's post was contradicting the statement you said earlier about the amount of fuel being exactly the same for every stroke, Integral. It isn't. If it were, an engine wouldn't be controllable. The amount of fuel being dumped into the engine per stroke is what the gas pedal controlls.

A car engine's power output should theoretically be the same in any gear at a given speed. Ie, if you are going 50mph and in shifting from 3rd to 4th your rpm goes from 3000 to 1500, the amount of fuel being burnt in each stroke should double, keeping the power output constant. The reason it isn't constant is mostly in drivetrain losses (friction), which also changes with rpm (with gear, I mean).

 

[Stingray]

Maybe it's time for Shawn to get a new transmission if he has to floor it in 5th gear to get to 50 km/h when he's barely touching the gas in 3rd, unless they number the gears in reverse order on Canadian cars.

Um, no. That's actually normal. It almost always takes more throttle to go a given speed in lower gears. You clearly have much less capacity for acceleration in higher gears (most of the time). The main exception is if you're hardly loading the engine at all no matter which gear you pick. No transmission failure I've ever heard of would cause that anyway.

Integral's statement about more fuel consumption with more acceleration also doesn't make sense. Of course the instantaneous burn rate increases when you add more throttle, but it also takes less time to reach a given speed. The change in kinetic energy is the same, so at lowest order, the total fuel consumed (as opposed to its max flow rate) isn't much different. Throttle plates are actually pretty inefficient when they're mostly closed, so a very slow acceleration is actually less efficient in most cases.

 

[Integral]

...

Now a lot of modern engines are getting smarter about feeding fuel so, the assumption of a constant a/f mixture may not be valid. With intelligent fuel metering the millage difference may be small.

So nobody read this?

OK, OK, So your your (Russ, and Average) I'll just leave it 3rd! :uhh:

 

[Stingray]

So nobody read this?

Your statement was that an engine takes in a predetermined amount of air at a given rpm, and that all that can be varied is the a/f ratio. That's not correct. Almost all cars are throttled mainly by adding a variable obstruction to the incoming air. Even if a/f were fixed, fuel consumption changes tremendously with throttle position. That ratio can only be varied from maybe 12:1 to 16:1 in any reasonable gasoline-powered engine. That's useful for getting a little more economy, but it's not huge.

 

[edward]

In 2008 the EPA is going to start fuel economy testing in the real world. Up to now they have used 55 MPH as there average highway speed.

That is why the window stickers and car ads always have the "your mileage may vary" disclaimer.

Engines get the best mileage at the optimum HP vs torque output for the individual car. Get the stats on your vehicle and keep the RPM in the appropriate range for the speed you want to maintain.

Diesels run in a vary narrow and low RPM range for example because they have a much higher torque output than gasoline engines.

If you really get tuned into the sound of your engine you can hear and feel when it is lugging or over revving.

your milage may vary

 

[ShawnD]

Cars are mostly designed to be most efficient in the highest gear while at normal highway speeds, but this characteristic is not intrinsically true of engines. See page three of this pdf.
http://www.oznet.ksu.edu/library/ageng2/L886.PDF

Note that the best fuel economy for a given power requirement does not always occur at the lowest engine speed. For an output of 50% of the maximum horsepower of the engine, the most efficient operating point is about 63% of the maximum RPM - at 102% of the minimum fuel consumption.

The most efficient operating point [fuel consumption/Hp]_min for the engine overall occurs at 58% of max power, and about 69% of max RPM. This would be the desired operating point for a hybrid's engine.

Note also another motivation for hybrids - the terrible inefficiency of the engine as we approach the no load condition. I see this as a huge advantage for hybrid drivers in large cities. In bumper to bumper, stop and go freeway traffic, the fuel savings should be tremendous. So I suspect that some hybrid drivers will get a much greater advantage than might be expected.

[Ironically, from an environmental engineering pov, it would make the most sense to allow standard vehicles to use diamond lanes, and make the hybrid drivers sit in traffic]
Late edits

Excellent post. This explains everything (especially the hybrid part)

 

[PA32R]

Someone needs to retake (or may be take?) a physics course.

Remember Newtons laws? F=ma?
Now re read my post and make a bit of an effort to understand it.

Argh, sorry, no. Let work be w, force be f, mass be m, acceleration be a, time be t, distance (or displacement) be d, speed be s, starting speed be s0, ending speed be s1, average speed be sa.

W=f*d (assume that force and distance are parallel)
f=ma so w=m*a*d, but this doesn't need to be used in this situation. Without loss of generality (as the mathematician likes to say) assume a constant acceleration from s0 to s1. The time taken to accelerate is (s1-s0)/a. With uniform acceleration, the distance traveled will be the average speed times the time. Again without loss of generality, let s0=0. Then sa=(s1)/2 and d=t*(s1)/2. Since mass and acceleration are constant, so is force, so for work, w=f*d=f*t*(s1)/2. Note that acceleration is not in the equation.

This also can be shown from the work energy theorem. A vehicle moving at, say, 60 m.p.h. has a specific amount of kinetic energy regardless of how quickly or slowly it got to that speed. It got that energy by having work performed on it. You're confusing work with the rate of doing work, otherwise known as "power."

You're correct that faster acceleration requires more force, but it's applied over a shorter distance. Work performed in getting up to a speed is independent of rate of acceleration. As to f=ma, that is true of course, but as shown above, for work it's w=m*a*d, and as a increases, d decreases proportionally so w is unchanged.

Consider that cow well milked,

Cheers!

 

[rcgldr]

It's a matter of engine efficiency for a given rpm, throttle position, and load factor.

engine efficiency = (power output) / (fuel consumption rate)

There's more internal friction (aerodynamic drag under the cylinders, drivetrain losses, ...) when rpms are higher, but this may be offset by how well the engine operates at at a moderate rpm versus low rpm (air+fuel circulation in and out of the cylinders above the piston).

A engine with a nearly flat torque versus rpm curve can operate well at any rpm above the rpm where the torque curve falls off. A high powered V8 might do well at 1500 rpm at 70 mph.

acceleration versus fuel consumption

The faster the acceleration, the more power required, but over a shorter period of time, and the work done is the same if you ignore the constant velocity related losses, such as aerodyanmic drag and internal friction. By accelerating to speed sooner, you spend a bit more time at the faster speed. If the speed is 45mph or less, fuel efficiency may be improved by accelerating faster than some minimal amount. If the speed is 70mph or more, the extra time spent at speed uses a bit more fuel.

Again it's an issue of engine efficiency. If you accelerate fast enough, where engine efficiency falls off at near maximum power, then you consume more fuel, but it's a shorter period of time, so if not much time is spent during acceleration, it's not going to affect fuel efficiency that much.

rpm versus fuel milage

Getting back to the OP, it depends on the car and the speed. At 70 mph, I'll get better milage in top gear on either my car or my motorcycle than in a lower gear. The motorcycle has higher power to weight ratio, but a lower weight to aerodyanmic drag ratio than a car, so aerodynamic drag is more of an issue on the motorcycle, and fuel milage is about the same in city or freeway (65 to 70 mph), and regardless of how fast I accelerate (about 35 mpg). The car on the other hand, gets much better milage on the freeway versus the city.

 

[NBAJam100]

Im sure the difference is small and you are still being more efficient if you drive with lower RPMs... but don't you also have to consider the fact that you are driving for longer when you are going slower, or driving with lower RPMS? As in, if just say I am doing 2k RPM for 4 minutes to get somewhere. If i was traveling the same distance but moving with 1k RPM I would be driving for longer, so although my RPM's are lower, I am doing more RPM because I am driving for longer correct?

 

[russ_watters]

Note, this thread is 2 years old...

 

[Cyrus]

It's amazing what's contained inside of books...

http://img8.imageshack.us/img8/396/curvem.png

Every one done speculating? Scanned for his and her pleasure.

 

[skeptic2]

With the engine turning over faster in a lower gear there will obviously be higher friction losses in the engine and transmission. I think these will be small but not negligible, certainly less than the ratio of the engine RPMs of the two gears. Why, because there are significant other losses that are the same at both RPMs such as rolling friction and drag.

So yes you will get lower gas mileage in a lower gear but probably not as much less as you'd expect.

 

[PA32R]

Note, this thread is 2 years old...

Yes, I'm sure I re-opened it. I stumbled on the thread by querying something or other in Google, and when I read the reply from Integral, I couldn't leave it alone. The conclusions (all else being equal, lower rpm's and relatively slow acceleration are better for fuel economy) were generally correct but his rationale for both cases was flawed.

Sorry if I've transgressed.

 

[PA32R]

It's amazing what's contained inside of books...

http://img8.imageshack.us/img8/396/curvem.png

Every one done speculating? Scanned for his and her pleasure.

I'd love to have such a map for my vehicle, but I can't find it. Modelling the optimal acceleration rate for a vehicle is fraught with complexity, the engine map would nail down one variable among many.

 

[Cyrus]

The drag at low speeds is very small.