mpg (US)
Miles per US gallon, the American fuel economy figure. Higher is better, and it is not comparable with UK mpg.
mpg (US) is miles traveled per gallon of fuel, using the US gallon of 3.785411784 liters. It is the figure on the EPA window sticker, with separate city, highway, and combined values, and higher means more economical.
The pitfall is the word "gallon". The imperial gallon used for UK figures is 4.54609 liters, about a fifth larger, so a car rated 35 mpg in the UK press scores only about 29 mpg in US terms while burning exactly the same fuel. A figure labeled just "mpg" is ambiguous, which is why CarPairly always names the gallon.
Converting from metric is a single division: mpg (US) equals 235.214583 divided by the consumption in L/100 km. Because the relationship is an inverse, gains at the top of the scale shrink: going from 40 to 50 mpg saves far less fuel per year than going from 15 to 20.
Differs by market
The US gallon is 3.785411784 L. The same car reads about 17% lower in mpg (US) than in mpg (UK), which uses the 4.54609 L imperial gallon; the UK figure is about 20% higher. Conversion: mpg (US) = 235.214583 ÷ L/100 km.