Running cost – Fuel
Fuel is usually the biggest day-to-day running cost of owning a car, so estimating it before you buy is worth a few minutes of maths. A fuel cost estimate combines three things: the car’s fuel economy (miles per gallon), your annual mileage, and the current price of petrol or diesel per litre. Please note that any estimate is exactly that – your true cost depends on your driving style and your car’s real-world MPG.
How the estimate works
The basic calculation is: annual miles ÷ MPG × 4.546 (litres per gallon) × price per litre. For example, a car averaging 45mpg driven 8,000 miles a year uses roughly 808 litres of fuel; at £1.40 per litre that is about £1,130 a year. Because pump prices change constantly, treat any figure as a snapshot rather than a promise.
Why official MPG figures flatter reality
Since 2017–2018, new cars in the UK have been tested under the WLTP procedure, which is more realistic than the old NEDC test but is still a laboratory cycle. Real-world economy is usually lower, sometimes considerably so – short journeys, cold starts, motorway speeds, roof boxes and aggressive acceleration all hurt MPG. Hybrids and plug-in hybrids show the biggest gap: a plug-in hybrid’s official figure assumes frequent charging, and owners who rarely plug in will see far worse economy.
When comparing used cars, check owner-reported real-world figures where you can, and remember that a poorly maintained car (clogged filters, under-inflated tyres, lazy servicing) burns more fuel. A car’s MOT and service record is a good clue to how well it has been looked after – you can review it with a car history check.