Fuel Cost Calculator

Estimating what a journey costs

Fuel cost comes from three numbers: distance, how much fuel the car uses, and what the fuel costs.

• MPG: fuel used = distance ÷ mpg
• L/100 km: fuel used = distance ÷ 100 × consumption

Multiply by the price per gallon or litre and you have the trip cost. Note that the two efficiency measures run in opposite directions — a higher MPG is better, a lower L/100 km is better.

Use your car's real consumption

Manufacturer figures come from standardised laboratory cycles and are optimistic in normal driving. The long-term average on your trip computer is a far better input, and it's usually 10–20% worse than the official number.

What actually moves consumption:

• Speed. Aerodynamic drag rises with the square of speed. Above roughly 70 mph, consumption climbs sharply.
• City driving. Constant acceleration and braking is the least efficient pattern there is.
• Load. Roof boxes and bike racks are expensive; a loaded roof rack can cost 10–25%.
• Tyre pressure. Under-inflation costs a few percent and is free to fix.
• Cold weather. Short winter trips can be dramatically worse, since the engine never reaches efficient operating temperature.

Splitting the cost

For shared journeys, the fuel figure is a defensible basis for splitting costs because it's verifiable. If you want to account for wear — tyres, servicing, depreciation — national mileage rates exist for exactly that purpose and run considerably higher than fuel alone.

Electric vehicles

The same structure applies with different units: kWh per 100 miles or km, priced per kWh. Home charging is typically a fraction of the cost of public rapid charging, so which one you'll actually use matters more than the vehicle's efficiency.