Fuel economy in automobiles

Fuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed.

There are no quantities or units for fuel usage defined in the International Standard ISO 80000 Quantities and Units, so the nationally-defined reciprocal quantities fuel economy and fuel consumption are used in this article.

Units of measure

MPG to L/100km conversion chart: blue: US gal, red: imp gal (UK)

The two most common ways to measure automobile fuel usage are:

Fuel consumption

The amount of fuel used per distance; Often an arbitrary distance, most commonly litres per 100 kilometres (L/100 km). This measure is used in Europe, China, South Africa, Australia and New Zealand. Canadian law allows for use of either litres per 100 kilometres or miles per gallon.^[2]

Fuel economy

Efficiency is defined as output per input. In automobiles it is the distance traveled per unit of fuel used; in miles per gallon (mpg) or kilometres per litre (km/L), commonly used in the UK, US (mpg) and Japan, Korea, India, Pakistan, parts of Africa, The Netherlands, Denmark and Latin America (km/L). If mpg is used the gallon should be identified, since an imperial gallon is about 20% larger than the US gallon.

Fuel economy and fuel consumption are reciprocal quantities. To convert either way between L/100 km and miles per US gallon, divide 235 by the number in question; for miles per imperial gallon, divide 282 by either number. For example, to convert from 30 mpg to L/100 km, divide 235 by 30, giving 7.83 L/100 km; or from 10 L/100 km to mpg (US), divide 235 by 10, giving 23.5 mpg. To convert between L/100 km and km/L, divide 100 by the number in question.

A related measure is the amount of carbon dioxide produced as a result of the combustion process, typically measured in grams of CO₂ per kilometre (CO₂ g/km). A petrol (gasoline) engine will produce around 2.32 kg of carbon dioxide for each litre of petrol consumed (19.4 lb/gal). A typical diesel engine produces 2.66 kg/L (22.23 lb/gal)^[3][4] though typically burns fewer litres per kilometre for an otherwise identical car. Since the CO₂ emissions are relatively constant per kilogram, they are proportional to fuel consumption.