---
title: "Fuel Economy Converter: MPG to L/100 km | Quiet Detour"
description: "Convert fuel economy between mpg (US), mpg (UK), L/100 km and km/L, with a comparison table and the mpg illusion explained with a worked example."
url: https://quietdetour.me/fuel-economy-converter/
---

mpg (US) · mpg (UK) · L/100 km · km/L

# Fuel economy converter

Type a fuel economy figure in any unit to see it in the other three. US and UK miles per gallon are different numbers for the same car, because the gallons differ.

## Two gallons and a litre

A US gallon is 3.785 litres and a UK (imperial) gallon 4.546 litres, so a car does about 20% more miles per UK gallon than per US gallon. Litres per 100 km turns the figure upside down: it is fuel per distance, so a lower number is better. km/L is distance per litre, the right way up again.

## The mpg illusion

Because mpg is distance per fuel, equal steps in mpg are not equal savings. Richard Larrick and Jack Soll showed in *Science* in 2008 that people judge fuel savings as if mpg were a straight line, and suggested quoting fuel per distance instead. Two car swaps over 10,000 miles show why:

| Swap (mpg US) | Fuel used | Fuel saved | L/100 km |
| --- | --- | --- | --- |
| 10 to 20 mpg | 1,000 to 500 gal | **500 gal** (1,893 L) | 23.5 to 11.8 (saves 11.8) |
| 25 to 50 mpg | 400 to 200 gal | **200 gal** (757 L) | 9.4 to 4.7 (saves 4.7) |

The 10-mpg gain saves 500 gallons; the 25-mpg gain, which looks bigger, saves only 200. In L/100 km the savings (11.8 against 4.7) line up with the fuel saved, which is why fuel used per distance compares cars more fairly.

## Comparison table

| mpg (US) | mpg (UK) | L/100 km | km/L | US gal per 100 mi |
| --- | --- | --- | --- | --- |
| 10 | 12.0 | 23.5 | 4.3 | 10.00 |
| 15 | 18.0 | 15.7 | 6.4 | 6.67 |
| 20 | 24.0 | 11.8 | 8.5 | 5.00 |
| 25 | 30.0 | 9.4 | 10.6 | 4.00 |
| 30 | 36.0 | 7.8 | 12.8 | 3.33 |
| 35 | 42.0 | 6.7 | 14.9 | 2.86 |
| 40 | 48.0 | 5.9 | 17.0 | 2.50 |
| 50 | 60.0 | 4.7 | 21.3 | 2.00 |
| 60 | 72.1 | 3.9 | 25.5 | 1.67 |

## Sources

- Larrick RP, Soll JB. The MPG Illusion. *Science* 2008;320(5883):1593–1594. [doi:10.1126/science.1154983](https://doi.org/10.1126/science.1154983)
- US gallon: 231 cubic inches, from [NIST Handbook 44](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition), Appendix C; with the inch defined as exactly 2.54 cm, exactly 3.785411784 litres.
- UK (imperial) gallon: exactly 4.54609 litres, from the [Weights and Measures Act 1985, Schedule 1](https://www.legislation.gov.uk/ukpga/1985/72/schedule/1).
- Mile: exactly 1.609344 km; 1 mpg (US) = 0.4251437 km/L, from [NIST Special Publication 811, Appendix B.8](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8).

## Questions

**Is 30 mpg the same in the US and the UK?**

No. The UK gallon holds more fuel, so the same car shows a higher figure in UK mpg: 30 mpg (US) is 36.0 mpg (UK), and 30 mpg (UK) is 25.0 mpg (US). Check which gallon a figure uses before comparing cars.

**How do I convert mpg to L/100 km by hand?**

Divide 235.215 by mpg (US), or 282.481 by mpg (UK). The same division works in reverse: 235.215 divided by L/100 km gives mpg (US). For km/L, divide 100 by L/100 km.

**Which unit is best for comparing cars?**

Fuel used per distance, such as L/100 km or US gallons per 100 miles, because halving it halves your fuel bill. With mpg, the same gain saves far more fuel on a thirsty car than on an efficient one.