Mixing US and imperial gallons

mpg is mpg, wherever the figure comes from.

The US gallon measures 3.785411784 litres, the imperial gallon 4.54609 litres. The same consumption therefore yields different mpg figures: 30 mpg US equals about 36 mpg imp.

Britain used the imperial gallon until 1965; the United States never adopted it. Vehicle tests from both countries still use their own gallon, often without indicating it in the abbreviation.

For any mpg figure check whether US or imp is meant — US for North American vehicles, imp for British sources. The difference is about 20 %. Use l/100 km for international comparisons, because that unit contains no gallon measure.