Work out your car's fuel efficiency from a fill-up, convert between UK MPG, US MPG, and L/100km, and calculate the cost of any journey.
Fuel efficiency is measured differently around the world. The UK and US both use "miles per gallon", but with different sized gallons, while most of Europe uses litres per 100km, where a lower number means better efficiency. This calculator converts between all three and works out what a journey will cost.
A UK (imperial) gallon is about 4.546 litres, while a US gallon is about 3.785 litres — roughly 20% smaller. Since MPG measures how many miles you travel per gallon of fuel, and a US gallon contains less fuel, the same car's real-world fuel consumption produces a lower MPG figure in the US measurement than in the UK measurement. A car that does 50 UK MPG will show as roughly 41-42 US MPG for exactly the same fuel use — neither figure is wrong, they're just different units.
Litres per 100km measures how much fuel you burn to cover a fixed distance, so a smaller number means you're using less fuel — the opposite relationship to MPG, where a bigger number is better because each gallon takes you further. If you're used to thinking in MPG, remember L/100km works the other way round: a drop from 8.0 to 6.0 L/100km is a real efficiency improvement, even though the number went down.
The fuel cost calculator takes your journey distance and your car's efficiency figure, converted internally to litres per 100km, to work out the total litres of fuel needed. It then multiplies that by your fuel price, converted to a price per litre regardless of whether you entered it per litre, per US gallon, or per UK gallon. The cost-per-mile and cost-per-km figures are useful for comparing journeys of different lengths.