How the fuel cost is worked out
UK cars are rated in miles per imperial gallon, but fuel is sold in litres. One imperial gallon is 4.546 litres, so the calculation is:
Litres used = miles ÷ mpg × 4.546
Cost = litres used × price per litre
For example, a 100 mile trip in a car doing 45mpg uses 100 ÷ 45 × 4.546 = 10.1 litres. At 174.5p a litre that is £17.63, or about 17.6p a mile.
Fuel cost per mile at different mpg
| Economy | Petrol at 174.5p | Diesel at 199.7p | Cost of 1,000 miles (petrol) |
|---|---|---|---|
| 25 mpg | 31.7p | 36.3p | £317 |
| 30 mpg | 26.4p | 30.3p | £264 |
| 35 mpg | 22.7p | 25.9p | £227 |
| 40 mpg | 19.8p | 22.7p | £198 |
| 45 mpg | 17.6p | 20.2p | £176 |
| 50 mpg | 15.9p | 18.2p | £159 |
| 55 mpg | 14.4p | 16.5p | £144 |
| 60 mpg | 13.2p | 15.1p | £132 |
| 70 mpg | 11.3p | 13.0p | £113 |
UK average pump prices for the week to 4 October 2026, from the government's weekly road fuel price statistics.
Why your real cost may be higher
The official mpg on a car's brochure comes from a laboratory test (WLTP) and most drivers get 10 to 20% less in daily use. Short trips with a cold engine, motorway speeds above 70mph, roof boxes, under-inflated tyres and air conditioning all push consumption up. For the most accurate answer, measure your own mpg by brimming the tank twice and dividing the miles by the litres added.
Splitting fuel costs with passengers
Sharing fuel costs with passengers is fine as long as you are not making a profit. If you charge more than the running cost, you may need a taxi or private hire licence and different insurance. Many drivers use the HMRC passenger rate or simply split the fuel bill, which this calculator does for you.
Sources
Rates and rules last checked 15 October 2026. These figures are a guide. For a decision that matters, confirm with the official source.