Questions and Answers

Statistical Data On Fuel Economy Q. Dear Vincent Ciulla, I'm a graduate student in Penn state university and major is statistics. Now I have a car's data set in order to analyze it. I want to know the difference among the following: City MPG (miles per gallon by EPA rating) Highway MPG RPM (revs per minute at maximum horsepower) Engine revolutions per mile (in highest gear)

I have already known their definition but, I don't know what is the important thing. Also, I want to know whether "Engine revolutions per mile (in highest gear)" is important or not and if it is important and what factors is related to it. Please answer me. Have a nice day.

Sincerely,
Daeyoung

A. If you read my article Understanding Fuel Economy Ratings, you will see what the difference is between Highway and City MPG and how it is determined by the EPA.

"Engine revolutions per mile (in highest gear)" has no significance to any type of measure.

Engine rpm does, however, make a big difference. It doesn't matter what kind of engine, high or low horsepower, gas or diesel, the higher the engine rpm, the more fuel it uses.

That was the reasoning a few years ago behind lowering the national speed limit to 55. At 55 mph the engine rpm is lower then at 65 mph, thus saving fuel. Fuel economy is the main reason behind fuel injection systems. They give the engine just the fuel it needs to operate at a certain condition and no more.

It is also why we now have automatic transmissions with a fourth (Overdrive) gear and lock up torque converters. Both of these lower engine rpm at higher speeds.