Petrol or diesel: what a used one actually looks like
From about 6 years old, the diesel standing on a UK forecourt has covered materially more miles than the petrol of the same age — between 21% and 44% more, consistently, at every age we hold enough of both to measure. The price gap runs the other way: diesels ask a clear premium when they are younger, and by the oldest cars the two are close to level. So on an older car you are often paying roughly the same money for a diesel that has done substantially more miles.
The same age, side by side
Every comparison of these two fuels argues about economy and tax. This one asks a narrower question that only someone holding stock can answer: when you stand in front of a petrol and a diesel of the same model year, what is different about them? Each row pairs cars of one age. We show a row only where we hold enough of both fuels at that age for a median to mean anything.
| Age | Petrol | Diesel | Mileage gap | Price gap |
|---|---|---|---|---|
| 6 years | 47,570 miles £12,985 | 57,370 miles £17,248 | +21% | +33% |
| 7 years | 51,121 miles £11,248 | 64,907 miles £14,323 | +27% | +27% |
| 8 years | 57,700 miles £8,497 | 75,000 miles £13,945 | +30% | +64% |
| 9 years | 59,000 miles £7,625 | 78,186 miles £9,998 | +33% | +31% |
| 10 years | 59,000 miles £6,995 | 83,910 miles £8,365 | +42% | +20% |
| 11 years | 62,226 miles £5,748 | 86,955 miles £6,945 | +40% | +21% |
| 12 years | 65,983 miles £5,588 | 90,000 miles £5,995 | +36% | +7% |
| 13 years | 72,175 miles £4,735 | 95,000 miles £5,480 | +32% | +16% |
| 14 years | 69,000 miles £4,500 | 93,000 miles £4,500 | +35% | 0% |
| 15 years | 80,672 miles £3,997 | 116,429 miles £3,695 | +44% | -8% |
Median mileage and median asking price for cars of each model year in our live stock, petrol against diesel, reference year 2026. Medians, not averages, so a handful of extreme cars cannot move a row. Asking prices, not sale prices — we hold what cars are advertised at, not what they eventually sell for. Ages where we do not hold enough of both fuels are left out rather than shown thin.
Why the mileage gap exists
It is not a statement about how diesels are driven so much as about which diesels end up for sale. Diesel was bought, for two decades, by the people covering the biggest annual distances — company drivers, commuters on motorways, anyone towing. Those cars arrive on the used market having done exactly what they were bought to do. The petrol of the same year was more often a second car or a shorter-journey car, and it arrives with less on the clock.
The practical consequence is that “petrol or diesel” is rarely a clean comparison in the real world. You are not usually choosing between two identical cars with different engines; you are choosing between a car that has done roughly 21% to 44% more miles and one that has not.
What actually decides it
Your journeys, not your annual total. A diesel wants long, steady runs. Its exhaust after-treatment needs to get hot to work, and a car used for short cold trips will keep presenting you with the bill for that. The same car doing motorway miles may be entirely trouble-free. Ten thousand miles a year in five-mile hops is a different proposition from ten thousand miles in fortnightly long runs, even though the number is identical.
Where you drive. For a diesel car the line in a clean air zone and in the ULEZ is Euro 6, not the fuel itself. The standard applied to new registrations from around September 2015, so age is a rough guide and not a safe one — check the specific car with our clean air zone checker rather than inferring it from the year.
The evidence, not the number. A high-mileage diesel with a full service history and a steady record of accumulation is often a better car than a low-mileage one that sat unused. Read the MOT history on GOV.UK for the shape of the car’s life, and how many miles is too many for how to judge a figure against an age.
What it costs to run. Fuel is only part of it. Vehicle tax for a used car depends on when it was first registered and on its emissions, which our car tax calculator works out, and the age and mileage your budget buys sets the context for everything above.