US MPG vs. Imperial MPG
A gallon is not the same volume in both systems.
By the TripFuel team · Updated November 3, 2026
Why the distinction matters
A US liquid gallon contains exactly 3.785411784 litres. An Imperial gallon contains exactly 4.54609 litres. Because an Imperial gallon is larger, the same car can travel more miles per Imperial gallon than per US gallon. A vehicle rated at 30 US MPG is about 36.03 Imperial MPG, even though the underlying fuel consumption has not changed.
Choose both units explicitly
Select the economy unit that matches the source of your MPG figure. Then select the volume unit used by your fuel price. A UK-style economy figure in Imperial MPG can be combined with a pump price per litre. A US economy figure usually uses US MPG and a price per US gallon. The calculator converts both into litres before applying the price, preventing a silent mismatch between the two gallon sizes.
A common source of error
Entering 30 Imperial MPG as 30 US MPG makes the vehicle appear less efficient than the intended figure. Merely changing the unit selector reinterprets the number you entered; it does not rewrite that number into an equivalent value. If you switch units while preserving the same vehicle, enter the equivalent economy figure as well. Review the fuel-used estimate to catch unexpectedly large changes before using the result for a trip budget.
Put the example into practice with the free fuel cost calculator.