How to work out fuel cost in the UK
Litres = gallons × 4.546
Cost (£) = litres × pence per litre ÷ 100
UK mpg uses the imperial gallon (4.546 litres), which is bigger than the US gallon (3.785 litres). A car doing 45 mpg in the UK does about 37.5 mpg in US figures, so make sure you use the UK number from your car's handbook or trip computer.
Fuel cost per 100 miles
| Economy | Litres per 100 miles | Cost at 140p/litre | Cost at 150p/litre |
|---|---|---|---|
| 30 mpg | 15.2 | £21.22 | £22.73 |
| 40 mpg | 11.4 | £15.91 | £17.05 |
| 50 mpg | 9.1 | £12.73 | £13.64 |
| 60 mpg | 7.6 | £10.61 | £11.37 |
Worked example
A 200 mile trip in a car doing 45 mpg uses 200 ÷ 45 = 4.44 gallons, or 20.2 litres. At 140p a litre that is £28.29, about 14.1p a mile. A return trip doubles it to £56.58, or £28.29 each for two people sharing.
How to use less fuel
- Keep to 60 to 65 mph on the motorway: fuel use rises quickly above 70 mph.
- Check tyre pressures monthly: under-inflated tyres raise consumption.
- Take roof boxes and bike racks off when you are not using them.
- Compare pump prices before a long trip: supermarket forecourts are often cheaper than motorway services.
For an electric car, the cost depends on your electricity price: see the electricity cost calculator.
Frequently asked questions
How much fuel does 100 miles use?
At 40 mpg, 100 miles uses 2.5 UK gallons, which is 11.4 litres. At 140p a litre that costs £15.91.
How do I convert mpg to litres per 100 km?
Divide 282.5 by the UK mpg. For example, 45 mpg is about 6.3 litres per 100 km.
Is UK mpg the same as US mpg?
No. The UK uses the imperial gallon of 4.546 litres, and the US gallon is 3.785 litres, so the same car shows a higher mpg figure in the UK.
How much does it cost to drive 1 mile?
In a 45 mpg car with fuel at 140p a litre, about 14p a mile in fuel alone.
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Last updated October 2026.