Fuel-cost formulas for MPG and L/100 km
The distance can represent one trip or the whole year.
Use kilometres for the distance and litres for the station price.
UK vehicles commonly display imperial MPG while stations sell fuel by the litre. In that mode, AutoPrisme converts gallons used to litres before applying the price.
Official fuel economy versus your real-world average
Published figures are useful for comparing vehicles on a consistent test. Your budget is better served by the average you actually observe over several fill-ups. Cold weather, congestion, short trips, speed, payload and tyre pressure can all change consumption.
Compare two cars with the same distance and fuel price
Keep annual distance and local price identical, then change only each vehicle’s real fuel economy. Fuel savings alone do not decide which car is cheaper: purchase price, finance, insurance, servicing, tyres and depreciation may outweigh them.
Add those lines with the total car cost calculator or compare a battery vehicle with the electric versus petrol calculator.
Sources and checks
AutoPrisme uses the figures you enter. The links below explain the rule, market context or consumer protections used to frame this calculator. Always verify the numbers in your own quote or contract.
- US Department of Energy — Vehicle Cost Calculator methodologyOfficial methodology multiplying annual fuel quantity by the selected fuel price.
- FuelEconomy.gov — Compare fuel economyOfficial US vehicle fuel-economy data for comparing published MPG figures.
- US Department of Energy — Vehicle Cost CalculatorA public reference tool using driving distance, MPG and fuel prices to estimate annual cost.
Frequently asked questions
How do I calculate annual fuel cost from MPG?
Divide annual miles by MPG to estimate gallons used, then multiply by the price per gallon. For UK MPG with a price per litre, convert each imperial gallon to 4.54609 litres first.
How do I calculate fuel cost with litres per 100 km?
Multiply the distance in kilometres by the L/100 km figure, divide by 100, then multiply the litres used by the price per litre.
Should I use official or real-world fuel economy?
Use your measured average when available. Official figures help compare vehicles, but traffic, temperature, speed, load and short journeys can change real consumption.
Does this include servicing, insurance or depreciation?
No. This calculator isolates fuel. Use the total car cost calculator to add finance, insurance, servicing, tyres, repairs and other recurring costs.