How does fuel economy change with speed?

Question

How does fuel economy change with speed? I'm specifically wondering about how the MPG of a Dodge Stratus four door changes with speed but I'd be curious about any vehicle. I'd especially be interested comparing engine types (gasoline, diesel, rotary, steam...).

If it's just RPM, why don't cars just come with higher gears so they can get good mileage at high speeds?

Answer 1

no, it's not just engine speed. the difference between the same car going 55 mph and requiring 26 horsepower to do it, you would think that it would take 52 horsepower to go 110 mph. simple logic tells you that to double your speed--requires that you need to double your horsepower. but, the truth is that it takes almost 4 times the horsepower to double your speed. Why--because you have to over come down force and drag. Yes, you need down force (the amount of tire contacting the road is about the size of a banana per wheel at 40 mph)---too increase as your speed comes up; otherwise the front of your car lifts and you go out of control. That is why all the front spoilers , wings, ect. Anyway --in order to produce that extra horsepower requires more fuel. Most cars have taller gears for highway speed--they are using peak toque (1600-2200 rpms) vs. peak horsepower (3600-5800 rpms) to push the car at highways speeds. So slowing engine speed helps-, but not the main factor.
Diesels have 30% more thermal energy in the fuel, rotary have one rotor doing the work of three pistons with out all the valve train and piston drag (so two rotors do the work of 6 cylinders), Stream--highly efficient to constant speed and long distance. but requires a huge amount of fuel to get going and to speed up.

Answer 2

The faster you go the higher the engine rpm is and the faster the rpm (revolutions per minute) is the more gas the engine needs to run faster. When your motor is running around 1800 to 2000 rpms you get your best mileage. When running higher rpms at greater speeds your going to use more gas for the motor to run at the higher rpms.

Answer 3

I don't know the science behind it, but from what I've heard in my economics class was that higher speed causes fuel to be used more quickly and less efficiently.

On the science, I believe that the faster the car is going, the more fuel it's pumping and the less time it has to use all that fuel and it thus is inefficiently pumping fuel. Don't take that as certain though, but I'm pretty sure that's what it is.

Answer 4

Simple, The faster your engine spins (RPM) the more fuel is required to make it spin faster. Thus reduces the miles you can get out of a gallon, since more gas is being used.

Answer 5

as many have said, and I agree. Higher the RPM's, the more fuel is required.
As far the gear ratio. If you change to higher gears, it would require even more fuel to get up to speed. (No low end torque)

Answer 6

The best mileage you get out of your car is going 55mph, thats with no stopping, like on highways/freeways. for every 5 miles over, your car comsumpts 10% more gasoline.