Litres per 100 km converts to imperial miles per gallon by division: mpg = 282.481 ÷ (l/100 km). The constant differs from the American one because the imperial gallon is twenty per cent larger, which is why British fuel figures always look better than American ones for the same car.
About the units
The imperial gallon is exactly 4.54609 litres, defined in 1824 as the volume of ten pounds of water. The US gallon is 3.785411784 litres, inherited from the English wine gallon of 1707. Britain kept miles per gallon after metricating fuel sales, so the pumps dispense litres while the car's economy is quoted in miles per imperial gallon — a genuinely mixed system that requires this conversion to make sense of. Note that both are efficiency units, so higher is better, while litres per 100 km is a consumption unit where lower is better.
The exact factor
The constant is 282.48093633, which is 62.137119 miles per 100 km times 4.54609 litres per imperial gallon. So 6 l/100 km is 47.1 mpg, and 50 mpg is 5.65 l/100 km. The relationship to American mpg is a straight ratio of 1.201: a car rated at 40 US mpg is 48 imperial mpg, and both describe identical fuel use. British motoring journalism quoting a figure without saying which gallon is a recurring source of confusion, and the twenty per cent gap is large enough to change which car appears more economical.
Where you meet this conversion
British and Irish vehicle data, where official fuel economy figures are published in miles per imperial gallon while the fuel itself is sold by the litre. Comparing a UK road test to a European manufacturer's l/100 km specification requires this conversion. Company car taxation, fleet policies and running-cost calculators in Britain all work in imperial mpg. Anyone reading a British review of a car also sold in America needs both this page and the US mpg one to compare the numbers.