Take a car travelling 300 miles on 10 US gallons. The measured value is 30 US MPG. Those 10 US gallons equal 37.854 litres. Dividing 300 miles by 37.854 ÷ 4.54609 gives 36.0 Imperial MPG. The vehicle did not become more efficient at a border; the larger Imperial gallon makes the distance-per-gallon number larger.
Worked example with numbers
Read the context before entering a value in a converter. A U.S. vehicle label, American receipt, and the site’s gas-mileage form use US gallons. A UK discussion may use Imperial MPG. Litres per 100 km avoids the gallon ambiguity, but only if the original MPG is converted with the correct gallon factor first.
How to make the calculation
The conversion path is transparent: US MPG × 0.425144 gives km/L; Imperial MPG × 0.354006 gives km/L. Or convert the fuel volume to litres before dividing distance by litres. The L/100 km converter uses the US-MPG constant, so it should not receive a UK MPG figure unchanged.
Check the units and assumptions
A fast plausibility check is that Imperial MPG is about 20% higher than US MPG for identical consumption. If a source reports 40 MPG and another reports 40 MPG without a country or litre value, they may not agree at all. Keep the unit in spreadsheets and notes: write “mpg (US)” or “mpg (Imp)” rather than relying on an unlabeled column.
Common mistakes
Do not convert a US gallon by multiplying by 4.54609 litres. Do not average a US-MPG trip with an Imperial-MPG trip without converting one of them. A further mistake is to call an Imperial gallon a metric gallon; litres are metric, but the Imperial gallon is a separately defined customary unit.
When this result does not apply
The gallon convention affects efficiency comparisons, not a fuel receipt denominated in litres. It also does not fix differences in test cycle, vehicle load, weather, tyre pressure, or driving speed. Use it to make like-for-like unit comparisons, then examine how the figures were measured.
Source for the external fact
The National Institute of Standards and Technology lists 1 U.S. gallon as 3.785412 L and the Canadian and UK Imperial gallon as 4.54609 L in its unit-conversion tables. Those named values are the basis for the difference, not a rounding convention.
Related calculators
Use the volume converter to inspect gallons in litres and the fuel-cost page once the consumption unit is unambiguous.