MPG to L/100 km

Type a figure with its unit: `30 mpg`, `7.8 L/100km`, `40 mpg UK` or `12 km/L`. It comes back in every unit at once, each with a copy button. A plain `mpg` is read as US gallons unless you change the setting, and a number on its own is read as US mpg.

For a figure from a European or Australian car brochure, type the L/100 km value and read off the mpg. For a UK car, add UK after mpg, since a UK gallon is bigger.

The two units run in opposite directions. Miles per gallon goes up as a car gets more efficient; litres per 100 km goes down. So the conversion is a division, not a multiplication: L/100 km = 235.215 ÷ US mpg, or 282.481 ÷ UK mpg.

Those constants come straight from the definitions. A mile is exactly 1.609344 km and a US gallon exactly 3.785411784 litres, so 1 US mpg is 0.425144 km per litre, and 100 ÷ 0.425144 = 235.215. 30 US mpg is 235.215 ÷ 30 = 7.84 L/100 km.

Because mpg is upside down, it misleads. In a study published in Science on 20 June 2008, Richard Larrick and Jack Soll showed that people expect fuel savings to grow evenly with mpg, and they don't. Going from 10 to 11.1 mpg saves about as much fuel over 10,000 miles as going from 33.3 to 50 mpg, roughly 100 gallons each. Swapping the least efficient car saves the most.

Litres per 100 km doesn't have the problem. It measures fuel used, so savings scale in a straight line: cutting 2 litres per 100 km saves 2 litres every 100 km, whatever the car.