Don't try using a British 1/2" wrench on an American 1/2" bolt.
At one time, the British gallon differed depending on what you were measuring and where you were measuring it. But, by the 19th century, two definitions had survived: the Queen Anne (or Wine) gallon, which was 231 cubic inches, and the Winchester (or Ale) gallon, of 282 cu in.
In 1824, the British abandoned both the Ale and the Wine gallons in favour of the Imperial gallon, based on the volume of 10 pounds of water (which works out at 277.41945 cu in). America, by this time, had already standardised on the Wine Gallon of 231 cubic inches (strictly speaking, this was defined as the volume of a cylinder 6 inches long and 7 inches in diamete, or, using the old approximation for pi, 231 cubic inches).
The result is that the US gallon is 83.267% of the British gallon. In more usable terms, the British gallon is about a fifth greater than the US gallon, and the US gallon is about 5/6 of the British gallon (or a little under 7 British pints)
