Free calculator
Petrol or diesel: which should you buy?
Diesel usually goes further on a litre but costs more per litre. Enter your yearly mileage and the two cars' mpg to see which is cheaper to fuel, and by how much.
Prices prefilled from the official Fuel Finder data, updated 28 August 2026. Change any box and the answer updates as you type.
The trade off in one line
Diesel engines are more efficient, so a diesel car typically returns noticeably more miles per gallon than the petrol version of the same model. But diesel costs more per litre, and the gap has been wide for years. Whether the better economy beats the higher price depends on one thing above all, how many miles you drive. The more you cover, the more the mpg advantage is worth. The fewer you cover, the less it matters and the more the dearer fuel and the higher purchase price count against it.
The prices are prefilled with today's typical UK figures from the official data and the mpg boxes with sensible defaults. Put in the real mpg of the two cars you are weighing up and your honest annual mileage for an answer that fits you.
What else to weigh
Fuel is only part of it. Diesels often cost more to buy and can cost more to service, and a diesel used mainly for short urban trips can suffer with its particulate filter, which is expensive to put right. Petrol suits low mileage and town driving, diesel suits high mileage and long runs. For a broader view of what each costs per 100 miles, including electric, see our guide to what it costs to drive 100 miles.
Common questions
Is diesel cheaper to run than petrol?
Per mile, usually yes, because diesel engines return more miles per gallon, but diesel costs more per litre so the saving depends on your mileage. High mileage drivers gain most. For low mileage and short trips petrol often works out cheaper overall.
How many miles a year make a diesel worth it?
There is no fixed figure, since it depends on the two cars' mpg and the price gap, which is why this calculator asks for your numbers. As a rule the case for diesel strengthens above roughly 12,000 to 15,000 miles a year and weakens below.
Are diesels bad for short journeys?
They can be. Modern diesels have a particulate filter that needs regular longer runs at speed to clean itself. A diesel used only for short urban trips can clog the filter, which is costly to fix, so petrol is the better choice for that kind of driving.