Fuel Economy Converter

Fuel economy is quoted three different ways around the world: miles per US gallon in America, miles per imperial gallon in the UK, litres per 100 kilometres across most of Europe, Canada and Australia, and kilometres per litre in much of Asia and Latin America. A car review from another country, a used import or a rental car abroad can leave you with a number you cannot compare. This converter turns any one of the four into all the others at once, using the exact definitions of the gallon, litre, mile and kilometre.

How to use the fuel economy converter

1. Type the figure you have, for example 30 or 7.8.
2. Choose what it is from the “Convert from” menu: US mpg, UK mpg, L/100 km or km/L.
3. Read the four results. The one you started with comes back unchanged, so you can use it as a sanity check.

If you want to know what a given efficiency costs in fuel, take the result to the fuel cost calculator; to work out your own figure from a fill-up, use the MPG calculator.

Fuel economy conversion formulas

• 235.215 = 100 km × 3.785411784 L per US gallon ÷ 1.609344 km per mile (all three are exact definitions)
• 1.20095 = 4.54609 L per imperial gallon ÷ 3.785411784 L per US gallon
• 0.42514 = 1.609344 km per mile ÷ 3.785411784 L per US gallon

The key thing to understand is that mpg and L/100 km are reciprocals. Mpg says how far you travel on one unit of fuel; L/100 km says how much fuel you burn over one unit of distance. That is why the conversion is a division, not a multiplication, and why small gains look bigger on low-mpg vehicles: going from 15 to 20 mpg saves 3.9 L per 100 km, while going from 40 to 45 mpg saves only 0.7 L per 100 km.

Worked example: a 30 mpg car in litres per 100 km

US mpg conversion table

Every value below is calculated from the formulas above and rounded for display.

US gallons, imperial gallons and why they differ

The US gallon is defined as 231 cubic inches, which is exactly 3.785411784 litres. The imperial gallon, still used for UK fuel economy figures, is defined as 4.54609 litres — the volume of ten pounds of water under standard conditions. Because the imperial gallon is larger, a car travelling the same distance on the same amount of fuel scores a higher number in miles per imperial gallon. When you read a UK review that says “45 mpg”, that is roughly 37.5 US mpg. Always check which gallon is meant before comparing two vehicles.

Official ratings also differ between test cycles. The US EPA label, the European WLTP figure and the older NEDC cycle all measure the same car differently, so a perfect unit conversion does not guarantee the numbers match on the road. Use the converted value to compare like with like, and treat real-world numbers from your own fill-ups as the best measure of your driving.

Sources

Unit definitions: NIST Special Publication 811 (Guide for the Use of the International System of Units) for the US gallon and international mile, and the UK Weights and Measures Act for the imperial gallon. The EPA’s fueleconomy.gov explains the US mpg label.