MPG

Miles per gallon, abbreviated mpg, is the fuel-economy figure used predominantly in the United Kingdom and the United States. It expresses how far a vehicle can travel on a single gallon of fuel, so a higher number indicates a more economical car. Because it directly answers the question most drivers actually ask, namely how far their money will take them, mpg remains one of the most widely quoted specifications despite the existence of more scientific alternatives.

The figure is derived by dividing the distance covered by the volume of fuel consumed over that distance, typically established under a standardised test cycle so that different cars can be compared on equal terms. Real-world economy almost always falls short of the quoted figure, because laboratory cycles cannot fully replicate aggressive acceleration, cold starts, heavy loads, headwinds or motorway cruising speeds, all of which raise consumption.

A persistent source of confusion is that the imperial gallon used in Britain is roughly twenty per cent larger than the US gallon: about 4.55 litres against 3.79 litres. As a result, a given car will always post a higher mpg figure in UK terms than in US terms, even though nothing about its actual fuel use has changed. Comparing figures across the Atlantic without converting them is therefore misleading.

Mpg is also an inverse, non-linear measure, and this catches many people out. Equal increases in mpg do not represent equal savings in fuel. Raising a thirsty car from 15 to 20 mpg saves far more fuel over a given distance than raising an already frugal car from 40 to 45 mpg, even though both improvements span five mpg. For this reason, many engineers prefer to think in terms of fuel consumed per fixed distance, such as litres per 100 kilometres, where the relationship with cost is linear.

The rise of electric vehicles has prompted an equivalent metric, MPGe, or miles per gallon of petrol-equivalent. This converts the electrical energy used into the energy contained in a notional gallon of fuel, allowing electric and combustion cars to be compared on a common energy basis, although it says nothing about running cost or local emissions.

Mpg sits within a family of related measures: litres per 100 kilometres, general fuel consumption, and ultimately carbon-dioxide output, which is closely tied to the volume of fossil fuel burned. Read sensibly, with its imperial-versus-US distinction and its non-linear nature in mind, mpg remains a useful and intuitive shorthand for a car's thirst.