A fuel economy converter changes a vehicle's efficiency between the units different countries use — miles per gallon, litres per 100 kilometres, and kilometres per litre. It is the conversion you need to compare a US, UK, Indian and European car on equal terms, since all four quote efficiency differently. This tool covers US MPG, Imperial (UK) MPG, L/100km, km/L and miles per litre.
The formula. There is a catch that makes this converter unlike every other one on the site: L/100km is inversely proportional to the others. MPG and km/L measure distance per fuel — bigger is better. L/100km measures fuel per distance — smaller is better. So you cannot multiply by a factor; you must divide. With kilometres per litre as the base, L/100km = 100 ÷ km/L, while 1 US mpg = 0.425144 km/L and 1 Imperial mpg = 0.354006 km/L. The two MPG figures differ because a US gallon is 3.785 L and an Imperial gallon is 4.546 L.
Worked example. A car rated at 30 US mpg does 30 × 0.425144 = 12.75 km/L. Converting to the European measure: 100 ÷ 12.75 = 7.84 L/100km. The same physical car would be advertised in Britain as 36 Imperial mpg — a 20% higher number for identical efficiency, purely because of the larger gallon. Halving consumption from 10 to 5 L/100km doubles the MPG figure from 23.5 to 47.0.
Where it's used. Comparing imported and domestic cars, budgeting fuel for a trip, and reading manufacturer specifications written for another market. The inverse relationship has a practical consequence worth knowing: improving a thirsty car from 12 to 10 L/100km saves far more fuel over the same distance than improving an efficient one from 6 to 5 L/100km, even though the MPG gain looks smaller — which is exactly why regulators increasingly prefer the L/100km form.