Fuel Efficiency

Fuel economy is quoted in mutually incompatible ways around the world, and two of them run in opposite directions. Miles per gallon and kilometres per litre improve as the number rises; litres per 100 kilometres improves as the number falls. Comparing a European specification with an American review therefore requires a real conversion, not a mental adjustment.

Gallons add a second trap. A US gallon is roughly 3.79 litres while an imperial gallon used in the UK is about 4.55, so the same car appears around twenty percent more efficient when quoted in imperial mpg. Anyone comparing a car review from one country with a manufacturer figure from another needs to know which gallon is in play.

This converter moves cleanly between mpg (US), mpg (imperial), kilometres per litre and litres per 100 kilometres, so you can compare vehicles on a single scale. Fleet managers, importers, car buyers and anyone budgeting a long road trip get a like-for-like number instead of an argument.

How to use the Fuel Efficiency

1. Select the unit your current figure is expressed in.
2. Choose the unit you want to compare against.
3. Enter the value from the specification sheet or your own trip computer.
4. Read the converted figure, remembering that lower is better for litres per 100 km.
5. Repeat for each vehicle so all your candidates sit on the same scale.

Examples

• A buyer comparing a US review quoting 32 mpg with a European brochure quoting 7.4 L/100 km.
• A fleet manager standardising a mixed international fleet onto a single efficiency metric for reporting.
• A driver planning a long trip converting a trip computer reading into litres per 100 km to budget fuel.

Frequently asked questions

Why do UK and US mpg figures differ for the same car?

Because the gallons differ. The imperial gallon is about twenty percent larger, so imperial mpg figures are correspondingly higher for identical consumption.

Is a higher number always better?

Only for mpg and km/L. For litres per 100 kilometres, a lower number means less fuel used, so lower is better.

Does this apply to electric vehicles?

Electric consumption is measured in kWh per 100 km or miles per kWh rather than volumes of fuel, so use an energy conversion instead.

Why is my real economy worse than the official figure?

Official figures come from standardised test cycles. Cold starts, short journeys, roof boxes, motorway speeds and heavy traffic all reduce real-world economy.

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