How to work out your fuel costs
Fuel is the most visible running cost of a car, and small changes in MPG or pump price add up over a year. This calculator turns your car's economy and the current price into a cost per mile, per journey and per year.
From MPG to cost per mile
UK cars quote economy in miles per gallon, but fuel is sold in litres. To convert:
Litres per mile = 4.546 ÷ MPG Cost per mile = Litres per mile × price per litre
So at 45 MPG and £1.40 a litre, each mile uses 4.546 ÷ 45 = 0.101 litres, costing about 14.1p.
Why you rarely hit the official figure
Manufacturers' MPG figures come from standardised lab tests (WLTP). Real driving — town stop-start, motorway speeds, air-con, roof boxes, cold starts and a heavy right foot — typically knocks 10–20% off. For an accurate cost, measure your own MPG: fill to the brim, note the mileage, fill again next time and divide miles by gallons used.
Practical ways to spend less
- Keep tyres correctly inflated — under-inflation wastes fuel.
- Remove roof bars and clutter to cut drag and weight.
- Ease off: steady speeds and gentle acceleration beat hard driving.
- Use a fuel-price app to find the cheapest local pumps; supermarket fuel is often cheaper.
Premium fuel rarely pays off
A 9,000-mile year at 45 MPG
Driving 9,000 miles a year at 45 MPG and £1.40 a litre costs about 14.1p a mile, or roughly £1,270 a year — about £106 a month — on fuel alone. Improving real-world economy to 50 MPG would save around £125 a year at the same price.
Common fuel-cost mistakes
- Trusting the official MPG. Real-world economy is usually 10–20% worse than the brochure figure.
- Ignoring price per litre swings. A 10p/litre change moves a typical annual bill by around £90.
- Forgetting fuel is only part of the cost. Insurance, tax, servicing and depreciation often cost more than fuel.
- Confusing US and UK MPG. A UK gallon is 4.546 litres; US MPG figures are about 20% lower for the same car.