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US vs imperial gallon: MPG difference

By RoadCost Editorial5 min readUpdated

A gallon is not always the same gallon. The US uses a gallon of 3.785411784 litres. The UK still uses the imperial gallon of 4.54609 litres for some fuel-economy figures. That difference is the reason a car can appear more efficient in Britain without changing at all.

Quick answer

One imperial gallon is about 1.20095 US gallons. A US gallon is about 0.8327 imperial gallons. Always state which gallon an MPG figure uses.

In this guide

  1. 1. The exact volumes
  2. 2. The MPG trap
  3. 3. The fuel-price trap
  4. Questions

Key takeaways

  • US gallon: 3.785411784 litres.
  • Imperial gallon: 4.54609 litres.
  • The same MPG number means different consumption in each system.

The exact volumes

The imperial gallon is about 20% larger than the US gallon. That is a large enough difference to matter in vehicle comparisons and fuel-cost work.

A mile is the same length in both systems, so the difference comes entirely from the gallon. When someone says 40 MPG, the next question should be: which gallon?

1 imperial gallon = 4.54609 litres
1 US gallon = 3.785411784 litres
1 imperial gallon ≈ 1.20095 US gallons

The MPG trap

40 MPG UK is about 7.06 L/100 km. 40 MPG US is about 5.88 L/100 km. The US figure represents lower fuel consumption, even though both numbers are 40.

This is not a trick or a marketing claim. It is a unit difference. Convert both figures to L/100 km before comparing them.

The fuel-price trap

Fuel prices follow the same split. UK prices are usually quoted per litre. US prices are usually quoted per US gallon. A US price per gallon cannot be compared directly with a UK price per litre without converting the volume.

If you are building a trip budget across countries, convert the price to a common unit first, then apply the fuel-economy figure in that same unit.

  • Convert gallons to litres before comparing prices.
  • Convert MPG to L/100 km before comparing cars.
  • Keep the gallon label visible in every calculation.

Questions

How much bigger is an imperial gallon?
It is 4.54609 litres, compared with 3.785411784 litres for a US gallon. That makes it about 20% larger.
Does the UK use imperial gallons for fuel?
UK fuel is sold in litres, but vehicle fuel economy has historically been quoted in miles per imperial gallon. Both conventions can appear in the same conversation.
Why do UK and US MPG differ?
The mile is the same, but the gallon is different. That changes how many miles one gallon represents.

Sources

Try the calculator

Fuel efficiency converter

Convert UK and US MPG without mixing the gallon definitions.

Related guides

MPG to L/100 km: exact conversion

Convert MPG to L/100 km with the right constant for UK and US gallons, including worked examples and common mistakes.

L/100 km to MPG: exact conversion

Turn litres per 100 kilometres into UK or US MPG with the exact formulas, examples and a simple way to remember the ratio.

Fuel economy vs consumption

MPG and km/L measure distance per fuel. L/100 km measures fuel per distance. Here is why the distinction matters for comparisons and costs.

A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: . Formulas are covered by automated tests. Prices are inputs, not live data.