Can I use rpm to compare gas mileage between cars?

Question

For example, if I know one car's engine is running at 3,000 rpm at 70 mph and another car's engine runs at the same rpm at 80 mph, can I tell the gas mileage of each at those speeds? This is a assuming a 1.5-liter 4-cylinder engine for both.

Answer 1

No, there's no direct correlation between engine RPM and fuel economy at a given speed.

I have two cars. At 75 MPH, one turns just over 2,500 RPM and gets about 25 MPG. The other turns nearly 4,000 RPM and gets 30 MPG. Engine sizes are similar.

Mass, rolling resistance, engine efficiency, and wind resistance are the primary determinating factors with fuel economy.

Answer 2

Fuel mileage is a very specific number that is only determined one way. How many miles you traveled divided by how many gallons you burned. You can't determine or compare it by RPM. Fuel mileage is, however, affected by an absolutely infinite buttload of variables, including, but not even close to limited to, engine RPM at a given speed, engine temperature, headwinds or tailwinds, the car's aerodynamics, its weight, the temperature of the air coming into the engine, tire pressure, tire design, road surface, altitude, humidity, fuel quality, fuel additives, fuel octane, fuel temperature, ignition, carburetor or fuel injection settings, the speed of the car, the grade of the road (uphill, downhill, or level) whether the air cleaner is new and clean or old and dirty, how many accessories are operating (like radios, air conditioning, lights, etc.), the kind of oil in the engine, and about a bazillion design differences between two seemingly identical engines. Because of design differences, some engines are more efficient (better fuel mileage) at lower RPM, some are more efficient at higher RPM. It also depends on how it's driven, with either a lead foot or an egg between your foot and the gas pedal.

Answer 3

if both engines are turning 3,000 RPM one is pushing the car 10 MPH faster this is all in the gear ratios. It would make sense that the one doing 80 is working a little harder than the one doing 70 so it would require a bit more fuel. The best way to calculate your average Gallon Per Mile is to write down your millage and Gallons filled every time you fill up the tank. Then you will be able to figure out howm many miles it took to suck down the so many gallons of gas required. Divide the difference in miles by the number of gallons it took to refill the tank, between your first and second fill up, then do it again on your second and third fill ups, and so on. you will notice an overall trend on your miles Per Gallon for each car.

Answer 4

No. There are too many other variables at work. You will have to actually keep track of the miles driven and divide by the fuel used.

Answer 5

If its the exact same engine or extremly close other wise you my as well just take out a calculater and look at the miles