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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. Overall fuel efficiency may vary per device, which in turn may vary per application fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.
In the context of transport, fuel economy is the energy efficiency of a particular vehicle, and is given as a ratio of distance travelled per unit of fuel consumed. Fuel economy is expressed in miles per gallon (mpg) in the USA and usually also in the UK-there is sometimes confusion as the imperial gallon is 20% larger than the US gallon so that mpg values are not directly comparable. In countries using the metric system fuel economy is stated in kilometres per litre (km/L) in the Netherlands, Denmark and in several Latin American or Asian countries such as India, Japan, South Korea, or as the reciprocal ratio, "fuel consumption" in liters per 100kilometers (L/100 km) in much of Europe, Canada, New Zealand and Australia. Litres per mil are used in Norway and Sweden.
Fuel consumption is a more accurate measure of a vehicle's performance because it is a linear relationship while fuel economy leads to distortions in efficiency improvements.
Weight-specific efficiency (efficiency per unit weight) may be stated for freight, and passenger-specific efficiency (vehicle efficiency per passenger).