MPG vs L/100 km: How to Convert Fuel Economy

Distance per Fuel vs Fuel per Distance

• MPG (miles per gallon) and km/L (kilometres per litre) are economy figures: how far you go on a unit of fuel. Used in the US, UK and some other countries.
• L/100 km is a consumption figure: how much fuel you burn over a fixed distance. Used across Europe, Canada, Australia and much of the world.

Because one is the inverse of the other, the relationship isn't linear, which matters more than people expect.

US vs UK Gallons

A UK (imperial) gallon is about 4.546 litres; a US gallon is about 3.785 litres. So the same car shows a higher MPG in the UK than in the US: 30 US MPG equals about 36 UK MPG. When comparing figures from different sources, check which gallon they mean.

Conversion Table

Values are rounded. To convert km/L to L/100 km, divide 100 by the km/L figure.

Why L/100 km Shows Savings More Clearly

Because MPG is distance per fuel, equal MPG gains don't save equal amounts of fuel. Consider driving 10,000 miles:

Going from 10 to 15 MPG saves far more fuel than going from 40 to 50 MPG, even though the second jump is bigger in MPG. This is often called the "MPG illusion". Consumption figures like L/100 km (or gallons per 100 miles, which US window stickers now also show) avoid it: savings scale directly with the difference.

The practical lesson: improving an inefficient vehicle saves more fuel than improving an already efficient one.

Quick Mental Conversions

• US MPG to L/100 km: 235 ÷ MPG. At 30 MPG, 235 ÷ 30 ≈ 7.8.
• UK MPG to L/100 km: 282 ÷ MPG. At 50 MPG, 282 ÷ 50 ≈ 5.6.
• US MPG to UK MPG: multiply by 1.2.
• km/L to L/100 km: 100 ÷ km/L. At 15 km/L, 100 ÷ 15 ≈ 6.7.

Converting in Formulas

If you calculate costs yourself, convert everything into one system first. For example, to estimate monthly cost with a UK MPG figure and a price per litre:

litres per mile = 4.546 ÷ UK MPG
monthly litres  = monthly miles × litres per mile
monthly cost    = monthly litres × price per litre

How to calculate fuel cost walks through full examples.

Let the Calculator Handle Units

The fuel savings calculator accepts MPG (US or UK), L/100 km or km/L, distances in miles or kilometres, and prices per litre or per gallon, and converts behind the scenes. That's useful when your car reports UK MPG but you think in kilometres, or when comparing figures from different countries.

Before relying on any figure, check how accurate it is; how to measure your car's real fuel economy explains why the trip computer and the official rating may both be optimistic.

Comparing Cars Fairly

When comparing two cars' fuel economy, check three things before trusting the numbers:

1. Same unit: convert both to L/100 km or the same type of MPG.
2. Same test standard: figures from different test cycles (for example US EPA ratings versus European WLTP figures) aren't directly comparable.
3. Same type of figure: combined, city and highway ratings differ a lot; compare like with like.

Then think in fuel used rather than MPG. If you drive 12,000 miles a year, a car that improves from 25 to 30 MPG saves 80 gallons a year (480 down to 400), while one that improves from 40 to 45 MPG saves only about 33 gallons (300 down to about 267).

Electric and Hybrid Equivalents

Electric cars are rated in different units, such as kWh per 100 miles or kWh per 100 km, and the US uses MPGe (miles per gallon equivalent) to compare them with petrol cars. These aren't directly comparable to MPG for cost purposes, because electricity and fuel prices differ; compare running costs instead.

Summary

Divide 235.21 (US) or 282.48 (UK) by one figure to get the other. Remember that L/100 km is consumption (lower is better) and shows real fuel savings more honestly than MPG.