Fuel Economy Converter

When to use it

Fuel economy is quoted in units that pull in opposite directions, which is why this converter earns its place. European listings give litres per 100 kilometres, where smaller is better; US listings give miles per gallon, where bigger is better; many trip computers show kilometres per litre. Comparing those figures by eye is hopeless, and doing it in your head invites an error in the wrong direction.

Use it to compare two cars advertised in different units, to check a rental company's quoted consumption, to test a review figure against the owner's manual, or to convert a number so it matches your own record. It accepts decimals and states plainly that its mpg figures are US, not imperial.

How the maths works

Everything is converted through kilometres per litre, the one unit where higher is better and the arithmetic is linear. Litres per 100 km become kilometres per litre as 100 ÷ value. US mpg become km/L as value ÷ 2.352145833, because a US gallon is 3.785411784 litres and a mile 1.609344 km. The reverse trips use the same constants.

The two headline units are reciprocal, not proportional, which is the most useful thing to remember here: 10 L/100 km is 23.52 mpg, and a 20% cut to 8 L/100 km raises that to 29.40 mpg, a 25% gain. Improvements always look better in mpg.

A worked example

A car uses 7 L/100 km. How does that compare with a rival quoted at 33 mpg?

1. Convert to kilometres per litre: 100 ÷ 7 = 14.2857 km/L.
2. Multiply by the mpg constant: 14.2857 × 2.352145833 = 33.60 US mpg.
3. Read the rival the same way: 33 ÷ 2.352145833 = 14.03 km/L, or 100 ÷ 14.03 = 7.13 L/100 km.
4. So the rival is about 1.8% thirstier (7.13 against 7.00), not better.

Common conversions

Read the table by column: the litres column grows to the right, the other two shrink. A car that looks dramatic in mpg can be unremarkable in litres per 100 km.

Common mistakes

• Mixing US and imperial gallons. Imperial mpg figures run about 20% higher for the same car, so a UK review and a US test can disagree while both are right.
• Assuming lower mpg is better. Only the litres figure works that way. Higher mpg means better economy.
• Converting as if the units were proportional. Doubling mpg does not halve litres per 100 km; the link is a reciprocal.
• Quoting the brochure number for a city commute. Cold starts and traffic can add 30% or more to the official figure.
• Typing zero or a negative value. There is no economy to convert at zero, so the tool asks for a value above zero.

Keep exploring

• Fuel Cost Calculator — turn a consumption figure into the cost of an actual trip.
• Unit Price Comparison — compare the price per litre at two filling stations or two pack sizes.
• Guide: Unit Conversion Mistakes to Avoid.