Imperial gallon vs US gallon

Both trace back to England, which is exactly why the split is confusing. Before 1824, England used several different "gallon" definitions depending on what was being measured: wine, ale, and grain each had their own. The American colonies took the wine gallon (231 cubic inches) with them, and the newly independent United States kept using it after 1776.

In 1824, Britain reformed its measurements and defined a single new imperial gallon as the volume of 10 pounds of water at a specified temperature, a bigger, cleaner unit that replaced all the old English gallons at once. The US never adopted the change, so it's still using the pre-1824 wine gallon today, more than two centuries after the split.