US MPG vs. UK Imperial MPG vs. L/100km: Complete Fuel Consumption Guide

North America and the UK express fuel efficiency as distance traveled per unit of fuel (Miles per Gallon, where a higher number means better economy). Continental Europe, Canada, Australia, and China express fuel consumption as volume burned over a fixed 100-kilometer distance (L/100km, where a lower number means better economy). Because one ratio is the mathematical reciprocal of the other, you cannot multiply by a constant—you must divide a constant by the input value.

• US MPG ↔ L/100km: L/100km = 235.214583 ÷ MPG (US) • UK Imperial MPG ↔ L/100km: L/100km = 282.480936 ÷ MPG (UK) • US MPG ↔ UK Imperial MPG: 1 MPG (US) = 1.20095 MPG (UK), because the 4.54609 L Imperial gallon holds 20.095% more fuel than the 3.78541 L US liquid gallon.

Even after converting UK Imperial MPG into US MPG using the 1.20095 gallon ratio, British and European window-sticker figures often appear 10% to 15% higher than North American stickers for the exact same engine. This occurs because the US Environmental Protection Agency (EPA) 5-cycle test includes aggressive highway acceleration, air-conditioning load, and cold-weather operation (20 °F), whereas the Worldwide Harmonised Light Vehicles Test Procedure (WLTP) uses different average speeds and thermal profiles.