Fuel Efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work.
In the context of transport, "fuel efficiency" more commonly refers to the energy efficiency of a particular vehicle model, where its total output (range, or mileage [U.S.]) is given as a ratio of range units per a unit amount of input fuel (gasoline, diesel, etc.). This ratio is given in common measures such as liters per 100 kilometers (L/100 km) (common in Europe, Canada and Australia) or litres per mil (Norway/Sweden) or miles per gallon (mpg) (prevalent in the USA, UK, and often in Canada, using their respective gallon measurements) or kilometres per litre (km/L) (prevalent in Asian countries such as India and Japan).