Fuel efficiency is form of thermal efficiency of the heat engine. Overall fuel efficiency may vary per device, also other factors can affect. Among them: weight category of efficiency for freight and passenger vehicles. There are many ways of expressing the fuel efficiency of motor vehicles, such as fuel consumption – it is the amount of fuel used per unit distance, fuel economy – it is the distance travelled per unit volume of fuel used. Some of the units, used for fuel economy: miles per gallon (m/g) is commonly used in the USA and in the UK; kilometers per liter (km/l) is often used in Russia, CIS countries, Denmark, in the Netherlands and several countries of Latin America and Asia.