Converting a gallon figure from a US car review or recipe into liters goes wrong if you don't know that US and UK gallons are actually different sizes.
US gallon vs UK gallon
A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters — about 20% larger. Always check which country's gallon a source is using.
Why there are two
The US standardized its own liquid measures independently, while the UK (and the former Commonwealth) uses the imperial system. That historical split is why both gallons still exist today.
Watch out when converting fuel economy
US mpg is based on the US gallon; UK mpg is based on the UK gallon — so the same "30 mpg" figure means different actual fuel economy depending on which one it is. Keep this straight when converting to km/L.
Gallons, quarts, and cups in US recipes
US recipes use the US gallon system's quarts, pints, and cups (1 gallon = 4 quarts = 8 pints = 16 cups). A US cup (about 236ml) differs from a metric 250ml cup, so convert carefully for accurate results.
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FastestUnitConverter's volume converter distinguishes US and UK gallons and handles liters, quarts, and cups all at once.
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