⛽ Fuel economy

Gas Mileage Converter

Switch between US and UK miles per gallon, kilometres per litre, miles per litre and litres per 100 km. Enter a figure, choose the units, then press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 28 October 2026 · Editorial standards · Report an error

About the Gas Mileage Converter

Fuel economy is unusual because one of its common units runs backwards: higher MPG and higher km/L are better, while a higher L/100 km figure is worse. The converter handles that inverse relationship automatically, so 30 MPG US becomes 7.84 L/100 km rather than a number that looks better when it is not.

A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres, which is why the same car advertises roughly 20% better MPG in the UK than in the US. The conversion here uses those exact definitions, not a rounded 3.8 or 4.5.

How the calculation works

1 US mile per gallon = 0.4251437075 km per litre.
1 UK (imperial) mile per gallon = 0.3540061899 km per litre.
1 mile per litre = 1.609344 km per litre.
Litres per 100 km = 100 ÷ (km per litre).

Reference values

Common fuel-economy figures across the three most-used scales
Fuel economy	US MPG	UK MPG	L/100 km
Very efficient hybrid	55	66.1	4.28
Small hatchback	40	48.0	5.88
Family SUV	28	33.6	8.40
Large pickup	18	21.6	13.07
Sports car	15	18.0	15.68

Worked example: 30 US MPG

A car returning 30 US MPG uses 7.84 L/100 km. The same car quoted in UK MPG is 36.0 MPG, because the imperial gallon is larger. At $3.50 per US gallon, 30 MPG costs $0.117 per mile, or $7.00 per 60 miles. At 7.84 L/100 km and $1.50 per litre, the same trip costs $7.06.

The two currencies and unit systems make direct comparison awkward, but the underlying fuel use is the same. Convert to litres per 100 km when comparing cars from different markets, and use the pump price in the same unit as the consumption figure. A 20% difference in MPG can be entirely a gallon-size difference.

Frequently asked questions

Why is UK MPG higher than US MPG?

An imperial gallon is 4.546 litres versus 3.785 litres in the US. The car uses the same fuel, but the larger gallon stretches the number by about 20%.

Is lower L/100 km better?

Yes. L/100 km measures fuel used per distance, so 5 L/100 km is more efficient than 10 L/100 km. It is the inverse of MPG.

Can I convert electric efficiency?

This page is for liquid fuel. For electricity, use kWh/100 km or mi/kWh; the arithmetic is a direct ratio, not an inverse one.

Why does UK MPG look better than US MPG?

An imperial gallon is 4.546 L versus 3.785 L in the US, about 20% larger. The car uses the same fuel; only the gallon used to measure it differs.

Sources and standards

This converter uses the exact US gallon of 3.785411784 L and the imperial gallon of 4.54609 L. Because mpg and L/100 km are reciprocal, the page shows the shape of the curve rather than implying a straight-line conversion between them.