Convert fuel economy

About the units

• Litres per 100 km (l/100 km) – Litres per 100 kilometres is the EU’s compulsory measure of fuel consumption. The car labelling directive requires the official figure to be given in litres per 100 kilometres to one decimal place, on a label of A4 size. The measure is built from two legal units and is therefore legal itself. Lower is better here, the opposite of miles per gallon. And the scale is not linear: going from 12 to 10 litres per 100 kilometres saves more fuel than going from 7 to 5.
• Litres per Swedish mile (l/mil) – Litres per Swedish mile rests on the Swedish mile of 10 kilometres, so the figure is always a tenth of the figure in litres per 100 kilometres: 0.5 l/mil is 5 l/100 km. It is everyday speech, mileage allowance and logbook rather than an official measure – the fuel label and the vehicle register follow the EU rules and state litres per 100 kilometres. The unit exists only in Sweden and Norway, and the word mile is a trap.
• Kilometres per litre (km/l) – Kilometres per litre measures the inverse of litres per 100 kilometres: distance per volume rather than volume per distance. The relation is therefore not linear but inversely proportional, 100 divided by the figure – 20 kilometres per litre is 5 litres per 100 kilometres, and the two measures meet at 10. Higher is better here. The unit is not the EU’s official measure but is common outside Europe. The trap is averages: mixed driving must be averaged over litres per distance.
• Miles per US gallon (mpg (US)) – Miles per US gallon is the American measure of fuel consumption. The US gallon is 3.785411784 litres and the mile 1,609.344 metres, which gives the conversion factor 235.214583: 7 litres per 100 kilometres is 33.6 mpg. The American fuel label shows city, highway and combined mpg, and now gallons per 100 miles as well – precisely because the mpg scale is inverted and non-linear. The trap is the gallon: the imperial one is 20% larger, so the same car scores higher in UK mpg.
• Miles per imperial gallon (mpg (UK)) – Miles per imperial gallon rests on the imperial gallon of exactly 4.54609 litres, fixed by the Weights and Measures Act 1985. That gives the conversion factor 282.480936: 7 litres per 100 kilometres is 40.4 mpg. The measure lives on in British everyday speech and the British motoring press, and printed car advertising must still give the official consumption in mpg as well as in litres per 100 kilometres or kilometres per litre. Here is the worst trap of the category: a 30 mpg (US) car is 36.0 mpg (UK).