How the estimate works
The DVLA records an official CO2 figure for most modern cars. Burning a litre of petrol produces about 2,392 g of CO2, and a litre of diesel about 2,640 g. Divide that by the car's grams per kilometre and you get its official fuel economy, which we convert to miles per gallon.
We then price a year of driving at the UK average of about 7,000 miles, using the average pump price from the government's weekly fuel price figures. For the week of 12 October 2026 that was 175.3p a litre for petrol and 200.0p for diesel. Add the yearly tax from our car tax check and you have a fair basis for comparing cars.
Real-world economy
Official figures come from standard tests, and real-world economy is usually a bit lower. Short trips, cold weather, heavy loads and an enthusiastic right foot all push it down. Treat the estimate as a way to compare cars on equal terms rather than a promise of what you will spend.
The gap between two cars is often more useful than either figure. If one car's estimate is £400 a year higher than another's, the difference in real life will usually be similar. If you drive much more or less than 7,000 miles a year, scale the fuel figure up or down. Your own habits make a difference too: smooth driving, correct tyre pressures and less weight in the boot all help.
Electric cars, hybrids and the full check
For electric cars we do not estimate charging costs, because they depend so much on your tariff and where you charge. You still get the tax figure, and electric cars are exempt from ULEZ and clean air zone charges. Petrol and diesel hybrids are estimated from their official CO2 figure, like any other car.
If you run the full history check, we use the car's combined mpg figure from its specification instead, where available. Either way, some costs sit outside the estimate: insurance, servicing, tyres and any work flagged in the MOT history. If the car will be driven in London, Birmingham or Bristol, check the clean air zone verdict too.
Put the figures to work
Running costs are where two similar cars can drift apart. A car with a higher CO2 figure usually costs more in fuel and, if it was registered between 2001 and 2017, more in tax as well, because the tax band depends on the same figure. Over three or four years of ownership, that difference can be worth more than a small saving on the price. Use the estimate when you shortlist, and again when you negotiate.