US gallon

Two countries, one word, a 20% difference. The US gallon is 3.785 L and the imperial gallon is 4.546 L, and the split happened because Britain reformed its measures after the American colonies had already adopted the older ones. Pre-1824 England had several gallons at once, sized for different goods: a wine gallon of 231 cubic inches, a larger ale gallon, and a corn or Winchester gallon for dry goods. The Weights and Measures Act of 1824 swept them away and defined a single imperial gallon as the volume of ten pounds of water at a specified temperature, which came out near 4.546 L. The United States, no longer taking instructions from Parliament, kept the wine gallon of exactly 231 cubic inches, and it is still exactly that today.

The consequences reach further than fuel prices. Every subdivision inherits the gap, so a US quart, pint and fluid ounce all differ from the imperial ones, and the fluid ounce differs in the opposite direction because the two gallons hold 128 and 160 ounces respectively. Canada, which uses litres, still sees imperial-gallon figures in older equipment and marine contexts, while American pump prices and tank capacities are always US gallons. When a specification says "gpm" for a pump, it is worth confirming which side of the Atlantic wrote it: the same number is a 20% different flow.