MPG vs L/100 km: Converting and Reading Fuel Economy

• MPG (miles per gallon) is distance per unit of fuel. Higher is better: the car goes further on each gallon.
• L/100 km (litres per 100 kilometres) is fuel per fixed distance. Lower is better: the car uses less fuel to cover 100 km.

Because one is distance ÷ fuel and the other is fuel ÷ distance, they are reciprocals. Doubling MPG halves L/100 km. That is why the conversion is "a constant divided by the value", and why the same formula works in both directions.

"MPG" is ambiguous. The gallon is defined differently in the two systems (NIST Handbook 44, Appendix C; UK Weights and Measures Act 1985, Schedule 1):

• US gallon = 3.785411784 L exactly (231 cubic inches).
• Imperial gallon = 4.54609 L exactly.

The Imperial gallon is about 20% larger (4.54609 ÷ 3.785411784 ≈ 1.201), so the same car shows a higher number in Imperial mpg than in US mpg. A figure from a US source and one from a UK source cannot be compared until both are in the same unit. If a figure just says "mpg", check where it was published before converting.

The constant is 100 × litres per gallon ÷ kilometres per mile, using 1 mile = 1.609344 km exactly. Rounded to three decimals:

A UK brochure lists a car at 52 mpg. UK figures use Imperial gallons, so:

282.481 ÷ 52 = 5.4 L/100 km.

Using the US constant by mistake would give 4.5 L/100 km — about 17% too low, making the car look more efficient than it is. In the other direction, a European figure of 6.5 L/100 km is 36.2 US mpg or 43.5 Imperial mpg.

Fuel used over a distance is distance ÷ mpg, so each extra mile per gallon saves less fuel than the one before. The table shows fuel for 10,000 miles at each US mpg value. Going from 10 to 20 mpg saves 500 gallons; going from 40 to 50 mpg saves 50 — one tenth as much for the same 10-mpg step.

L/100 km does not have this problem: fuel used is proportional to it, so a 1 L/100 km reduction saves the same fuel over the same distance whatever the starting point. This is the main reason MPG comparisons are easy to misread.

1. Put both figures in the same unit — and, for MPG, the same gallon.
2. Prefer comparing in L/100 km (or gallons per 100 miles), because differences there are proportional to fuel used.
3. Compare figures from the same test procedure. A laboratory test figure and a figure you measured yourself are not directly comparable.

Example: a 25 US mpg car (9.4 L/100 km) against a 35 US mpg car (6.7 L/100 km). The difference is 2.7 L per 100 km, which you can multiply by your own distance.

• From L/100 km: litres = km × L/100 km ÷ 100. 450 km at 6.5 L/100 km = 29.25 L.
• From MPG: gallons = miles ÷ mpg, in the same gallon as the mpg figure. 280 miles at 40 US mpg = 7 US gallons = 26.5 L.

To measure your own consumption, fill the tank, note the distance driven until the next fill, and divide the fuel added by that distance. Use the gallons to litres and miles to kilometres converters for the unit steps.

This guide stops at fuel quantity. Turning fuel into money, or comparing a petrol car with an electric one, is covered in the EV vs petrol running cost guide. Sources and assumptions used on this site are listed on the methodology page.