Miles per gallon (MPG) is a unit of measurement that measures fuel economy in automobiles, that is, how many miles a vehicle can travel on one gallon of fuel. It is used similarly in United States and the United Kingdom, although the U.S. gallon is about 83% of the Imperial gallon used in the UK.
One should note that MPG works differently than litres per hundred kilometres. l/100 km denotes a rate of fuel consumption, while MPG is a measure of fuel economy (or 'gas mileage'). If a car uses less fuel, the MPG increases, and l/100 km decreases, but the percentages will not match, because the values are reciprocal.
Because consumption is an inverse function of MPG, MPG can be a misleading indicator of fuel efficiency gains. People intuitively take the difference in MPG when comparing two cars. This leads them to underestimate the savings from small improvements on low MPG cars
A measure of "Gallons Per Mile" (GPM), such as gallons per 100 miles, provides an accurate view of consumption for a given distance of driving. Unlike MPG, the GPM of one car can be subtracted from the GPM of another car to get a direct measure of fuel savings.