The Imperial gallon is a unit of volume in the imperial system of units, where the US gallon is used exclusively in the United States. The imperial gallon is 20% larger than the US gallon. The gallon is commonly used to measure volumes of fuel for vehicles. Miles per gallon are a unit often used for fuel consumption. This means that countries like Canada will quote a different fuel economy for a car than an American ad for the same car (often used to the car company's advantage).