Fuel cost formulas
Metric: cost = km × (L/100 km) ÷ 100 × price per liter
US: cost = miles ÷ mpg × price per gallon
Example
A 350 km drive in a car that uses 6.5 L/100 km needs 350 × 6.5 ÷ 100 = 22.75 liters. At 1.85 per liter, that
costs about 42.09. A round trip doubles it to 84.18.
Ways to save fuel
- Slow down a little: most cars use noticeably more fuel above about 90 km/h (55 mph).
- Keep tires properly inflated: under-inflated tires increase rolling resistance.
- Remove roof boxes and racks when you don't need them.
- Drive smoothly: gentle acceleration and anticipating stops can save 10–20%.
Frequently asked questions
How do I calculate the fuel cost of a trip?
Work out how much fuel the trip uses, then multiply by the price. In metric: distance (km) × consumption (L/100 km) ÷ 100 × price per liter. In US units: distance (miles) ÷ mpg × price per gallon.
How do I convert mpg to L/100 km?
Divide 235.21 by the US mpg figure. 30 mpg (US) is 235.21 ÷ 30 ≈ 7.8 L/100 km. For UK (imperial) mpg, divide 282.48 by the mpg figure instead.
Why does my car use more fuel than its official rating?
Official figures come from standardized lab tests. Real driving with higher speeds, air conditioning, heavy loads, cold weather and city traffic usually uses 10–30% more. For a realistic estimate, use the average shown on your car's trip computer.
How should we split fuel costs on a road trip?
Splitting evenly between everyone in the car is the simplest. Enter the number of people and the calculator shows each person's share.