gallongal·lon /ˈgælən/ noun [countableC]1British EnglishBrE a unit for measuring liquids, equal to 4.5435 litresA price increase of 2p on a litre of petrol is equal to a rise of more than 9p a gallon.
a 12.6 million-gallon cargo of industrial oil
2American EnglishAmE a unit for measuring liquids, equal to 3.785 litresThe average price of unleaded gasoline has now risen to over $3 a gallon.
An improvement in car mileage of just one mile per gallon would save 350,000 barrels of crude oil per day