How to work out the fuel cost of a journey
Gallons used = miles ÷ mpg. A UK (imperial) gallon is 4.546 litres, so litres used = miles ÷ mpg × 4.546.
Fuel cost = litres used × price per litre.
Cost per person = fuel cost ÷ number of people. Choose a return journey to double the distance.
If you know your economy in litres per 100 km instead, the calculator converts miles to kilometres (1 mile = 1.609 km) and works out the litres directly.
Worked example 1: a 100-mile trip
120 miles, 45 mpg, petrol at £1.40 a litre
Litres = 120 ÷ 45 × 4.546 = 12.12 litres
Cost = 12.12 × £1.40 = £16.97
Cost per mile ≈ 14p
Worked example 2: a shared return trip
200 miles each way, 50 mpg, £1.40 a litre, 2 people
Total distance = 400 miles; litres = 400 ÷ 50 × 4.546 = 36.37 litres
Cost = 36.37 × £1.40 = £50.92
Each person pays £25.46
Worked example 3: an electric car
120 miles each way, 3.5 miles per kWh, home charging at 26p per kWh, 2 people
Energy = 240 ÷ 3.5 = 68.57 kWh
Cost = 68.57 × £0.26 = £17.83 (before charging losses), or £8.91 each
At a public rapid charger costing 75p per kWh the same journey would cost about £51.
What affects your fuel economy
- Speed: driving at 80 mph uses noticeably more fuel than 70 mph.
- Weight and drag: roof boxes, bike racks and a heavy boot all increase consumption.
- Tyre pressures: under-inflated tyres increase rolling resistance.
- Short trips and traffic: a cold engine and stop-start driving use more fuel than the official combined figure suggests.
- Cold weather: electric cars lose range in winter, partly because of cabin and battery heating.