There are many ways to express the fuel efficiency of vehicles, such as fuel consumption which is the amount of fuel used per unit distance and fuel economy which is the distance travelled per unit volume of fuel used.
Several representative units are used for fuel efficiency; such as miles per gallon (mpg) which is commonly used in USA and UK, or kilometers per liter (km/L) which is commonly used in Denmark, Netherlands and in several Latin American or Asian countries.
Fuel efficiency is a form of thermal efficiency. The overall fuel efficiency may vary from different device. Other factors also can affect the vehicle's fuel efficiency, such as weight-specific efficiency for freight and passenger-specific efficiency.