# Does engine RPM affect gas mileage?

turbo
Gold Member
You're missing the point, Cy. You can easily run engines over-rich or over-lean at any RPM (with possible detrimental effects to fuel-consumption and/or engine life). The idea that an engine MUST consume a certain amount of fuel at a certain RPM regardless of load, ignition timing, etc is a gross over-simplification. I've been tweaking ICEs for about 40 years now. If you don't believe me, ask a mechanic.

When I bought my old Wide-Glide, I was getting about 40-45 mpg. After fitting it with better-scavenging pipes, low-restriction air filter, high-flow petcock and filter, and rebuilding the S&S Super E racing carb with a Yost Power-tube for improved atomization, I could get 50+ mpg riding through the mountains 2-up, and the bike ran like a scalded cat. Many of the parameters that I had to adjust manually on that bike can be tweaked through mapping control modules these days. BTW, getting reasonable low-speed performance out of a large-bore butterfly carb is somewhat of an art, which is why you see many older modded H-Ds smoking on acceleration at low RPM. Air-speed across the main-jet venturi is insufficient to atomize the fuel properly, so it is not burned completely. Nothing wrong with the design - just poor skills on the part of the person who tuned the bike and thinks that oversized jets are the answer for performance gain.

You're missing the point, Cy. You can easily run engines over-rich or over-lean at any RPM (with possible detrimental effects to fuel-consumption and/or engine life). The idea that an engine MUST consume a certain amount of fuel at a certain RPM regardless of load, ignition timing, etc is a gross over-simplification. I've been tweaking ICEs for about 40 years now. If you don't believe me, ask a mechanic.

When I bought my old Wide-Glide, I was getting about 40-45 mpg. After fitting it with better-scavenging pipes, low-restriction air filter, high-flow petcock and filter, and rebuilding the S&S Super E racing carb with a Yost Power-tube for improved atomization, I could get 50+ mpg riding through the mountains 2-up, and the bike ran like a scalded cat. Many of the parameters that I had to adjust manually on that bike can be tweaked through mapping control modules these days. BTW, getting reasonable low-speed performance out of a large-bore butterfly carb is somewhat of an art, which is why you see many older modded H-Ds smoking on acceleration at low RPM. Air-speed across the main-jet venturi is insufficient to atomize the fuel properly, so it is not burned completely. Nothing wrong with the design - just poor skills on the part of the person who tuned the bike and thinks that oversized jets are the answer for performance gain.
Turbo, I thought the y-axis of the map (PSI) was for a given load. The map tells you for a given load and RPM you need z- amount of fuel.

I agree with what you said about leaning or making it rich. But that's really not where the engine was designed to operate at.

I assume the PSI is manifold pressure like in an airplane.

turbo
Gold Member
I would think that variable ignition timing would be a good way to improve performance and change that map. Still, I don't think it is something you would want to turn on and off. If you can get better performance, why turn it off?
It is NOT something you would want to turn off, Russ. You still want variable ignition timing (not the old vacuum advance type), but the sweet part of the Power Commander is that you can change the slope of that timing advance to optimize acceleration or perhaps to sacrifice some performance for fuel efficiency. Unless you really want to dig into the maps, and figure out what's going on, the tweaking of the ignition timing is hidden from the user. You are faced with choices like what kinds of performance, economy, etc you want at what loads and rpms, and the web-site suggests maps that might help you get there, based on what kinds of modifications you might have made to the bike. There are tons of maps, tailored to specific combinations of exhausts, intakes, etc. It might take a few tries to get the responsiveness, torque, etc you're looking for and it's a hell of a lot cheaper to program your own module than to run back to H-D and fork over \$ to remap your stock module.

Pythagorean
Gold Member
Cy, turbo,

For instance, would you use the same chart for a diesel or rotary engine? It says in the subscript that it's for a V-8 engine. Is it much different for a V-6 or a straight six?

my intuition tells me it should be different for a diesel engine, but I really have no idea.

The chart I gave is only valid for the engine that made it. But it gives you a general idea of whats going on.

turbo
Gold Member
Cy, turbo,

For instance, would you use the same chart for a diesel or rotary engine? It says in the subscript that it's for a V-8 engine. Is it much different for a V-6 or a straight six?

my intuition tells me it should be different for a diesel engine, but I really have no idea.
It can be very different, Pythagorean. The firing angle can affect the efficiency tremendously, and that depends greatly on the physical configuration of the engine. The vibration and long-term wear posed by unbalanced firing angles can be quite detrimental to engine life.

Ranger Mike addressed some of these variations here:

If you have a well-balanced engine firing at at well-controlled intervals, you can make them lighter and more powerful than competing designs, and they will last longer. I have a lot of experience with H-D engines and since they are staggered 2-cylinders, they tend to lope at some points. Still, they are fun to tweak. I could never launch off the line like the crotch-rockets, but when I could sell a 10-year-old bike for several thousand more than I paid for it after making years of incremental improvements, it was pretty nice.

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Pythagorean
Gold Member
It can be very different, Pythagorean. The firing angle can affect the efficiency tremendously, and that depends greatly on the physical configuration of the engine. The vibration and long-term wear posed by unbalanced firing angles can be quite detrimental to engine life.

Ranger Mike addressed some of these variations here:

wow, that's a pretty intense packet of information!

turbo
Gold Member
wow, that's a pretty intense packet of information!
He's got some practical application knowledge under his belt, and is a pretty reliable source. I have tweaked bikes much more than autos, and I gladly accept his advice on the latter. The principles are the same - the applications differ little in practice, though sometimes it's nice to have a lower-mass vehicle to tweak.

I have a friend (local guy) who has captured the US drag-racing title in his category. Darn! Who'd have thought that a 340 Duster could show you its oil pan off the line and beat out the competitors to the traps?

PA32R
Just to clarify, when you say "rolling resistance", is that where the tires meet the ground only or does it include all drive losses?
Only tires meeting pavement. It treats the vehicle as a "black box."

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.

A lot of people get testy when old threads are re-ignited and I have no idea why. It would be like claiming that Newton or Faraday is obsolete and should not be brought up any more.

At certain RPM's if the car is tuned well there is an effect from the exhaust which improves the mileage/performance of the car similar to a turbo charger. It is called the scavenging effect where a pulse wave sucks air into the engine on the intake side, all the way from the exhaust side while the valves are open at the right positions. Engine tuning books have information on this. Therefore, RPM is not always directly related to mileage.

That being said, I have found with the majority of cars that keeping my foot on the gas peddle as absolutely little as possible, accelerating really slowly, and keeping RPM's at about 1800-2100 on highway (85-95km/h) does improve mileage significantly. Going below 1800 RPM usually lugs the engine and I find my foot becoming heavier to maintain speed.

I have read going above 85-95km/h starts to cost more mileage due to air resistance also.

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I have read going above 85-95km/h starts to cost more mileage due to air resistance also.
Rule of thumb given in driving class was about 10% drop every 10km/h.

It's nice to see people are still thinking about this kind of thing. I still own the car I had when I started this thread and it still seems to get very consistent mileage regardless of what I'm doing. Last fillup was roughly 48mpg UK (40mpg US). All city miles during off hours (no rush hour driving). Pedal right to the floor until I get up to speed, then set cruise control

I drive a '97 Toyota Starlet automatic(with a home-brewed cold air intake, anyway..). Even though it's automatic, you are actually still much in control of engine rpms by how you use the throttle and switching overdrive on/off.

I drive to and from school a total of 40 miles a day and here's my observation: It doesn't matter to the mpg whether I'm driving like a hormone-crazed teen(keeping high rpms by flooring it) or driving like an old lady(the previous owner of my car is one!).

What does seem to matter to mpg is the speed I chose to cruise at the motorway(freeway, Interstate, w/e). Some of your got this correct!

Also keep in mind, that car engines are usually most efficient at their peak torques when the Helmholtz Resonance is in full effect (in a beneficial manner). Peak torques of gas engines for cars are by no means low rpm. They are usually 4000 rpm or higher.

Helmholtz Resonance is btw, like a 'free boost' to your engine without added fuel consumption so it improves efficiency. It is good for efficiency, however, since it's at a relatively high rpm, it can increase wear on the engine if you always to try to run at peak torque rpm.

Office_Shredder
Staff Emeritus
Gold Member
Last fillup was roughly 48mpg UK (40mpg US).
Is this some exotic relativity effect that I'm unaware of, or are gallons just smaller in the US?

Kurdt
Staff Emeritus
Gold Member
US gallons are smaller.

FlexGunship
Gold Member
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.
Man, what car do you drive?! From 1st to 6th, my car is always trying to hit 120mph.

I get significantly better mileage by modulating pedal use in higher gears. I have an engine-tuner that feeds back realtime injector pulse-width data. By looking at the PWM signal and figuring in the RPM at the time it can also display fuel consumption on a "per mile" basis.

To my knowledge, this is the most accurate possible way to find fuel mileage and I see a difference between 55mph in 5th and 55mph in 6th. I can't possibly call it a scientific test, but I get about 16mpg in 5th and 19-10 in 6th. That's NOT trivial.

Furthermore, I also know at which speed my car is most fuel efficient. Sadly, it turns out to be about 75-80mph.

FlexGunship
Gold Member
I don't understand what you mean by 'loading the map you want'. The map decribes a physical system. For x RPM and x PSI you NEED y fuel flow.
Engines often run in non-stoichiometric configurations. Check the voltage of your O2 sensor and you'll find that... WOAH... it's not always giving you a 14.7:1 ratio!! Honda is famous for running their engines super lean at low RPM and low load. This allows them to get crazy gas mileage numbers during highway driving.

EDIT: and I totally load different maps. At the track I have one that keeps the engine rich and ready for hard driving. For daily driving I switch back to something closer to stoichiometric, and for winter (i.e. now) I have a fuel-miser map (because I don't do a lot of hard driving in the winter).

I get up to 29 MPG average in my 'vette by using cruise control. If I make a say, 30 mile drive, very robustly, I'll be lucky if my average doesn't fall below 17. It absolutely makes a difference.

Jasongreat
The easiest way, ime, to get the best mileage in a gasser is to install a vacuum gauge, the lower you can keep the vacuum the better the mileage. In a diesel, keeping the pyrometer at the lowest number you can, will do the same thing.

...keeping the pyrometer...
"pyrometer?"

Sorry, but that's not in the SAE manual.

I get up to 29 MPG average in my 'vette by using cruise control.
And I averaged 47 mpg in my 1979 VW diesel. Back in 1979. That was more than thirty years ago for those who can't count.

We're fooling ourselves if we think we're somehow making "progress" on this front. Please stop fooling yourselves. That milage is better than what you'll enjoy from a Toyota Prius.

If you want to make progress, http://www.aptera.com/".

Grow a brain, folks! Here on PF, we actually HAVE brains, so please use them, and let's begin with 100 mpg or better, ok? Thanks. We owe it to both ourselves as well as the other several billion.

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Jasongreat
I guess I will have to go take the one out my truck cause its not in the SAE manual then. On a diesel, measuring the heat of the exhaust gasses tells you how much fuel is being injected. Higher the temp, the more fuel you are burning.

I guess I will have to go take the one out my truck cause its not in the SAE manual then. On a diesel, measuring the heat of the exhaust gasses tells you how much fuel is being injected. Higher the temp, the more fuel you are burning.
Sounds like you're talking about an EGT gauge, to me. (exhaust gas temperature) Pyrometer is an archaic usage, so if you have an archaic diesel, you might want to leave it in!

Just kidding with you...

On a more serious note, other "-ometers" originating in and around the 1700s include barometer, ceilometer, chronometer, cyclometer, ergometer, Fathometer, gasometer, geometer, hydrometer, hygrometer, kilometer, manometer, micrometer, odometer, pedometer, photometer, rheometer, seismometer, spectrometer, speedometer, tachometer, thermometer, viscometer...

And some of those we still use, so the idea of someone calling an EGT gauge a "pyrometer" is by no means archaic. Just different than what I'm used to hearing.

Yalcrab
My car gets does exactly 2000 RPM at 50 MPH and 3000 RPM at 75 MPH.
For a 75 mile non-stop trip using cruise control the engine does same number of revolutions at either speed.
It just takes less time at 75 MPH, so the amount of fuel should be the same.
It also does 4000 at 100 MPH, but I'm not that eager to get a ticket.

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FlexGunship
Gold Member
My car gets does exactly 2000 RPM at 50 MPH and 3000 RPM at 75 MPH.
For a 75 mile non-stop trip using cruise control the engine does same number of revolutions at either speed.
It just takes less time at 75 MPH, so the amount of fuel should be the same.
I don't know of any cars that use a "constant" fuel curve. Usually the injector pulse width gets wider through the power band. It's likely that you're burning more gas at higher RPM. Furthermore, wind resistance will play a significant role at those speeds.

EDIT: Hmm, did I encourage a necropost? It's been 9 months.

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