Fuel Economy Converter

About the Fuel Economy Converter

This converts a vehicle's fuel efficiency between the systems used around the world — miles per gallon (US and UK), litres per 100 kilometres, and kilometres per litre. It's the tool for comparing a car's economy figure against a spec sheet in another system, or making sense of a US mpg rating in European L/100km terms.

Need to convert the fuel volume itself, or a trip distance? See the Volume Converter or the Length Converter.

How it works — why it's not a simple factor

Fuel economy is measured two opposite ways, so the conversion is inverse, not a straight multiplier:

• Distance per fuel — mpg and km/L: higher is better.
• Fuel per distance — L/100km: lower is better.

Because of that, L/100km = 235.215 ÷ mpg(US) (or use 100 ÷ km/L). The reciprocal relationship means the numbers don't scale linearly — improving from 20 to 40 mpg roughly halves your L/100km, but from 40 to 60 mpg saves much less fuel per mile. The tool also distinguishes US mpg from UK (Imperial) mpg, because the two gallons differ.

Assumptions and behaviour

• US mpg and UK mpg are separate. The Imperial gallon is about 20% larger, so the same car reads higher in UK mpg than US mpg.
• The inverse relationship means better economy (more mpg) gives fewer L/100km, and vice versa.
• Results show about 7 significant figures.
• The Swap button reverses the two units.

Limitations

• US mpg ≠ UK mpg — a common source of confusion when comparing cars across markets. Always check which gallon a figure uses; this tool lists both.
• Because the conversion is reciprocal, a value of 0 (or a blank) can't be inverted, so extreme or zero inputs may show a dash or an unhelpful result.
• It converts a stated economy figure; it doesn't calculate economy from distance travelled and fuel used.
• Real-world economy varies with driving conditions — any converted figure is only as accurate as the rating you enter.

Privacy

The conversion runs entirely in your browser. Nothing is uploaded or stored.