Fuel Consumption (L/100 km)

Most of the world measures fuel the sensible way round: litres consumed per hundred kilometres driven, so a smaller number is better and the quantity is directly proportional to what you pay. Divide the fuel you put in by the distance you covered since the last fill, scale to 100 km, and you have a figure you can compare against the sticker.

The inverse convention used in the United States creates a genuine cognitive trap known as the MPG illusion, documented by Richard Larrick and Jack Soll in Science in 2008. Improving a vehicle from 10 to 15 mpg saves more fuel over the same distance than improving one from 30 to 40 mpg, which almost nobody believes on first hearing. In litres per hundred kilometres the same comparison is obvious at a glance: 23.5 falls to 15.7, a saving of 7.8, while 7.8 falls to 5.9, a saving of 1.9. The metric unit makes the arithmetic honest.