Diesel vs petrol: which is cheaper to run?
The per-mile comparison once the pump price gap, the economy advantage and the ownership costs are all counted.
6 min read · Written by Sam Richards · Last reviewed 2026-08-17
Key takeaways
- Diesel usually costs several pence per litre more than petrol but returns 15-25% better economy.
- Diesel tends to win above roughly 12,000 miles a year of mostly longer journeys.
- Short urban trips are the worst case for a modern diesel because of particulate filter regeneration.
The pump price gap
The UK diesel premium moves around with wholesale markets and has been anywhere between a couple of pence and more than twenty pence per litre in recent years. We track the current gap live so you can use today's number rather than a remembered one.
Because diesel engines are more thermally efficient, a diesel car doing 55mpg against a comparable petrol at 45mpg still comes out ahead per mile at a modest price premium.
Working it out for your mileage
Convert both to pence per mile. Divide the pump price per litre by your real-world mpg divided by 4.546. Do it with your own observed economy rather than the official figure, because the gap between the two is larger for diesels.
Then add ownership costs. Diesels typically carry higher purchase prices, dearer servicing, and in urban areas charges such as the London ULEZ for older models. Those fixed costs are recovered only through mileage, which is why annual distance is the deciding variable.
- Under about 8,000 miles a year — petrol almost always cheaper overall.
- 8,000 to 12,000 miles — close; depends on journey type and purchase price.
- Above 12,000 miles of motorway work — diesel usually wins clearly.
The urban diesel problem
Modern diesels need periodic diesel particulate filter regeneration, which requires sustained higher-speed running. A car used only for short cold trips can fail to complete regenerations, leading to blocked filters and expensive repairs.
If most of your driving is short and urban, petrol or an EV is the better mechanical fit regardless of what the fuel-cost arithmetic says.
Frequently asked questions
Is diesel cheaper to run than petrol?
Per mile, often yes for higher-mileage drivers, because the 15-25% economy advantage usually outweighs the price premium per litre. For low annual mileage the higher purchase and servicing costs win out.
Why is diesel more expensive than petrol in the UK?
Duty is the same on both, so the difference is wholesale: UK and European refineries produce less diesel than the region consumes, so more is imported and priced against global demand from freight and heating.
How many miles a year justify a diesel?
As a rough guide, above 12,000 miles a year of predominantly longer journeys. Below 8,000 miles the fuel saving rarely covers the extra ownership costs.
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Prices come from the UK Government's Competition and Markets Authority (CMA) open fuel price scheme and refresh throughout the day. Figures older than seven days are excluded.