Efficiency in engineering terms is a comparison between the actual performance of a system and the theoretical maximum performance (which is usually greater).

Common measures of efficiency in vehicles include:

• thermal efficiency – the ratio of useful work produced by an engine to the fuel energy it consumes
• volumetric efficiency – the ratio of the intake charge volume (at standard pressure and temperature) in the combustion chamber compared to the actual volume of the chamber
• transmission efficiency – the ratio of the energy output to the energy input

Efficiency is often expressed as a percentage, or a number between 0 and 1. In engineering calculations efficiency is usually represented by the Greek letter eta (η).