Amusingly it's a different gallon in the UK, while it's mostly obsolete it's the volume of ten pounds of water or about 4.5 litres.

Fuel has been sold by the litre for many years, but it was deemed too expensive to send someone round changing all the road signs to km/h and kilometres. Fair enough in my view but we then made the absolutely daft decision to keep measure fuel efficiency in miles to the gallon, not miles to the litre or even litres / 100 miles. Confusing to Britons who don't use gallons for anything else, confusing to other Europeans who've been entirely metric for generations, and confusing to Americans who use a different gallon.