Fuel Economy Converter

Two ways to measure fuel use

Fuel figures come in two opposite forms. Economy units say how far you go on a fixed amount of fuel — miles per gallon, kilometres per litre — so a bigger number is better. Consumption units say how much fuel a fixed distance takes — litres per 100 km, gallons per 100 miles — so a smaller number is better. Most of Europe, Australia and Canada quote L/100 km; the US and UK quote mpg, but with different gallons.

Because the two forms are reciprocals, you can't convert between them by multiplying by a single factor the way you convert miles to kilometres. This converter turns your number into kilometres per litre first, then into every other unit, and shows both steps.

It only converts the rating. The fuel cost calculator takes a rating plus a distance and a price and tells you what a trip will cost; for plain distances and volumes, see the length and volume converters.

Conversion formulas

All conversions rest on three exact definitions:

1 mile

1.609344 km (international mile)

1 US gallon

231 cubic inches = 3.785411784 L

1 imperial gallon

4.54609 L (UK)

The most common conversions then follow directly:

The constant 235.2146 is 100 × 3.785411784 ÷ 1.609344, and 282.4809 is 100 × 4.54609 ÷ 1.609344. Because a UK gallon is about 20% bigger than a US gallon, the same car shows a higher mpg figure in Britain than in the US. The same formula works in both directions: 235.2146 ÷ L/100 km gives US mpg.

Worked example

A car rated at 30 mpg (US):

1. Kilometres per litre: 30 × 1.609344 ÷ 3.785411784 = 12.7543 km/L
2. Litres per 100 km: 100 ÷ 12.7543 = 7.84 L/100 km (the shortcut 235.2146 ÷ 30 gives the same)
3. UK mpg: 12.7543 × 4.54609 ÷ 1.609344 = 36.03 mpg (UK)
4. Miles per litre: 12.7543 ÷ 1.609344 = 7.93 mi/L
5. Gallons per 100 miles: 100 ÷ 30 = 3.33 gal/100 mi

Going the other way, a car using 7 L/100 km gets 235.2146 ÷ 7 = 33.60 mpg (US), or 40.35 mpg (UK).

Why mpg differences mislead: the MPG illusion

Fuel used over a distance is distance divided by mpg, so each extra mile per gallon saves less fuel than the one before. Richard Larrick and Jack Soll showed in Science (2008) that people tend to treat mpg gains as if savings grew in a straight line, and so undervalue improving the least efficient vehicles. They suggested expressing efficiency as fuel per distance, such as gallons per 100 miles.

Compare two upgrades over 100 miles:

• 10 mpg → 20 mpg: 10 gallons becomes 5 gallons, saving 5 gallons.
• 30 mpg → 50 mpg: 3.33 gallons becomes 2 gallons, saving 1.33 gallons.

The second upgrade looks twice as big (20 mpg against 10), yet the first saves 3.75 times as much fuel. In litres per 100 km the picture is obvious: 23.52 → 11.76 is a cut of 11.76 L, while 7.84 → 4.70 is a cut of 3.14 L.

Things to keep in mind

• Check which gallon. British and older Canadian figures use the imperial gallon. A “50 mpg” UK figure is about 41.6 mpg in US terms.
• Official ratings are test-cycle results. Real consumption depends on speed, traffic, load, weather and driving style, and ratings from different test cycles are not directly comparable even after conversion.
• Electric vehicles are rated in energy per distance (such as kWh/100 km) or in mpg-equivalent, which this converter does not handle.