# Optimum speed for mileage per gallon

This chart was done a few years ago on my 1993 Chrysler Intrepid:

http://www.davesbrain.ca/miscpix/mileage.gif

What I find odd about it is:
- the lack of curvature. I would have thought it would level out to the left.
- the minimal effect of the AC. It has a smaller effect than changing highway speed by 20kph.
If it was a medium to small 6 cyl then I believe it, but in my 99 neon (2.0 dohc 4 cyl) if I turned the air on it nearly halved my mpg at highway speeds.
Ive noticed for some reason 6 cyls have less of a problem with the air on.

Thanks for the info. With limited automobile mechanical knowledge, my common sense failed me. So the best solution for both of my questions is to drive as slow as practical in the most efficient gear. (I'll carry a neckbrace in the glove compartment for when I get rear-ended.) But at the lower gears it doesn't make sense to drive revved up if I can give it a little more gas and get to a more efficient gear, true?

mgb_phys
Homework Helper
Basically on a freeway you drive as slowly as possible in the highest gear.
More important is to avoid unnecessary braking and acceleration.
And if you can avoid driving a 4ton SUV to take one small kid to school that would help.

Basically on a freeway you drive as slowly as possible in the highest gear.
Click and Clack have dealt with this at length on Car Talk. (You may have never heard of them- think of Benny Hill and Monty Python with engineering degrees and doing a car show on the radio. ) They recommend your solution, i.e., driving as slowly as possible in the highest gear, without the engine "lugging", which typically means perhaps 1500 rpm or so.

Danger
Gold Member
I'm not sure about the math aspect of it. My Roadrunner (650hp full race 440) got about 18 mpg at 59 mph. At 60, the secondaries on the carb started to open, and it dropped to about 14 mpg. In city stop & go traffic, or if I put my foot in it on the highway, it was about 4 or 5 mpg.
So my opinion is... it entirely depends upon the vehicle.

mgb_phys
Homework Helper
or if I put my foot in it on the highway, it was about 4 or 5 mpg.
Have you thought of converting it to just burn dollar bills - might be cheaper!

I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.

Danger
Gold Member
Have you thought of converting it to just burn dollar bills - might be cheaper!
I'm sure that it would be... but we don't have dollar bills. Smelting Loonies and trying to feed them though the carb before they harden is more of an engineering challenge than I care to tackle.

I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.
True to some extent. If your revs are below your power band, however, you 'lug' the engine. That makes for lousy mileage.

Last edited:
But remember!!! It depends on the load as well. Just because I'm at 3000 rpm doesn't mean anything gas-wise. The fuel spent is constantly adjusted even at the same rpm. So if you fo uphill at the same speed, the rpm stays the same, but more work is needed to turn over the engine, so the throttle/fuel increases to keep it at the same rpm. Opposite with downhill.

Same applies to adding a trailer to the back. Same RPM, different fuel expenditure.

If you toss around words like "average" though, then you can draw some correlation of course. It becomes an efficiency thing with the power band, and I'd assume its a simple function of:

Wind Drag(Function of Speed) + Drivetrain Loss(Should be a constant) + Efficiency of Engine(Function of RPM, therefor due to locked gear, say 5th, a function of Speed) + Loss due to grade of road

I figure any weight of the vehicle at a constant speed only comes into play for the drivetrain friction loss.

The drag/drivetrain can be easily approximated. But there is no "function" for the power curve of an engine, you'd just have to map it using a dyno.

If you assume drag goes like : -kv^3
And drivetrain friction is a multiplier on the output of the engine : D*Out
And the Output of the engine is linear, then your best efficiency is at lowest velocity.

but if your engine power output is some sort of peak @ some rpm, then its possible that at that peak you get a better MPG than along the -kv^3 line.

Last edited:
From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.

So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.

But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts

Danger
Gold Member
peak torque which depends on the power band of your camshafts
Not just the cam. Intake and exhaust runners also help to determine where the pipe is. Ideally, all three should be tuned to the same range.

brewnog
Gold Member
From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.

So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.

But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts

This all assumes that combustion airflow is proportional to the engine speed. This is not the case.

From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.

So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.

But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts
This brings to my mind an interesting question. What if peak torque was in the higher RPMs? I am aware that every car has it's differences in powerband, however lets say that we have a car that has peak torque performance at 6100 RPM (2007 Honda Civic Si). I do believe that driving at this peak would result in poor fuel economy.
I am beginning to realize more and more that fuel economy is very much related to the amount of work done per unit fuel(gallon).
I think that this further effects our definition of fuel economy.

Last edited:
Mech_Engineer
Gold Member
I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.
It takes 2 metrics to determine how much fuel is going into the engine (or 3 if you don't assume AFR=14.7):

1) Engine RPM

2) Engine Manifold Pressure

Engines modify the amount of vacuum they are pulling (or in the case of forced induction, vacuum and boost) by opening the throttle plate and/or controlling the pressure being forced into the engine. Theoretically, at full throttle you're running at or close to 0 inhg of vacuum (14.7 psi absolute at sea level), and most engines will pull about -20 inhg (-10 psi) vacuum with the throttle plate closed (4.7 psi absolute). By modifying the density of the air going through the engine, you can modify the amount of power being made while at a set rpm range. Because of these two competing metrics, it's possible to make the same amount of power and use the same amount of fuel at different rpms.

Take for example you're going up a hill and you can maintain constant speed at either full throttle in 4th gear using 0 psi vacuum (14.7 psi absolute) at 2000rpm, or you can shift into 3rd gear and use partial throttle at -7 psi of vacuum (7.7psi absolute) at 4000rpm. Both states have the same amount of air going through the engine and therefore you are making the same amount of power at AFR=14.7, assuming of course the volumetric efficiency of the engine is constant through the rpms range, which it might not be... Essentially no matter how many rpms you are pulling, you have a specific power requirement that has to be met through burning gas for energy. This is true of going up a hill, or going against wind resistance at 75mph.

So, the best mileage will generally be where you minimize the amount of air going into the engine AND the load on your engine while maximizing the gear reduction between the engine and the wheels (highest gear). Generally, I have found that most vehicles get peak mileage at 55-60mph.

The opitmum speed is ZERO or was that too obvesious.

Knowing how car makers like to work the figures the optimum will be at whatever speed the Government decides to test at.

With diesel engines it is a little easier to do the calculations as the makers publish fuel consumption figures based on full load across the rev range. By looking at both full consuption and HP it is then possiable to select optimum revs for an application or more to the point the right engine (yes this is based on stationary engine applications) With drag being the square root of speed the answer becomes one of the right revs V excess speed. In trucking aplications I think they aim to maintain RPM between 1400 & 1800 for best power and fuel.

I always thought that the best speed for cars and other light vechciles was 44 MPH.

What fuel ecomony does come down to though is the weight that you are trying to move and the aerodynmics. Afterall a 3 tonne SUV is always going to take more fuel to get from A to Z than a 1.25 tonne hatchback.

oil change, good tires, adult driving will all add up to the mileage of your family car, you can also use bio-fuel save money ! ( I read an article, guess, popular mechanics that one company in CA has introduced a bio-fuel generator to be used in back yards, cost $10K. the price is expected to be$6K soon, uses leaves and bushes

Rick

Last edited by a moderator:
DaveC426913
Gold Member

mgb_phys
Homework Helper
Without the extra weight of clothes you get better mileage!