Three common fuel-economy units
US MPG measures miles traveled per US gallon. Imperial MPG uses the larger Imperial gallon, so the same car receives a higher Imperial MPG number. L/100km measures liters consumed over 100 kilometers. Higher MPG means better economy, while lower L/100km means better economy.
Useful conversion formulas
Imperial MPG to L/100km: 282.480936 ÷ Imperial MPG.
Because the relationship is reciprocal, a change of five MPG does not always produce the same change in L/100km. Compare the converted fuel consumption rather than comparing the number difference alone.
Worked example
A vehicle rated at 30 US MPG converts to about 7.84 L/100km and 36.0 Imperial MPG. Another vehicle rated at 6.5 L/100km converts to about 36.2 US MPG. The second vehicle uses less fuel under the stated test figures.
Estimate fuel cost separately
Fuel economy is not fuel cost. To estimate a trip, convert the driving distance into the matching unit, calculate the fuel quantity, then multiply by the current local price. Include a realistic buffer for traffic, weather, hills, luggage, and driving style.
Ratings versus real use
Official tests make cars easier to compare, but real consumption varies. Vehicle condition, tire pressure, speed, temperature, heating or air-conditioning, route, and load can all matter. Use the same source and test standard when comparing two models.
Try the KokoroFlow Fuel Economy Converter to switch between all three units.