Petrol vs Diesel Running Costs
Diesel costs 21.2p a litre more than petrol, but a diesel engine typically returns 15% to 25% better economy - so the running costs are closer than the pump price suggests. At the current averages of 161.2p for petrol and 182.4p for diesel, a 45mpg petrol car and a 55mpg diesel cost almost exactly the same per mile.
Cost per mile, side by side
| Economy | Petrol at 161.2p | Diesel at 182.4p |
|---|---|---|
| 30 mpg | 24.4p | 27.6p |
| 40 mpg | 18.3p | 20.7p |
| 45 mpg | 16.3p | 18.4p |
| 50 mpg | 14.7p | 16.6p |
| 55 mpg | 13.3p | 15.1p |
| 60 mpg | 12.2p | 13.8p |
Read it across rather than down. A petrol car at 45mpg costs 16.3p a mile. A diesel matching that needs about 51mpg. Diesels commonly do achieve that, which is why the two often end up level.
The diesel premium is unusually wide right now
Diesel carries exactly the same 52.95p fuel duty as petrol. The 21.2p gap is entirely wholesale: diesel demand is driven by freight and agriculture, and the UK imports a significant share of its diesel, leaving it more exposed to global distillate markets.
Historically the gap has been nearer 5p to 10p. At 21.2p the economics currently favour petrol more than they have for much of the last decade.
A year of widening
The diesel premium is not a constant, and the last twelve months have stretched it dramatically. A year ago diesel cost under 8p a litre more than petrol; the gap now stands at 21.3p. On a 55-litre fill that is the difference between a £4.35 and a £11.72 premium for the same tank.
For anyone running the numbers on a future purchase, the lesson is to treat the spread as a moving part rather than a constant. Over the past year it has been under 8p and it now stands at 21.3p; a parity calculation made in one regime does not survive the other.
The cause is not tax - diesel and petrol carry identical duty and VAT - so the widening lives entirely in the wholesale market and retailer margins, which our fuel prices guide unpacks. The practical consequence for this comparison is that the mileage at which a diesel's better economy repays its higher fuel cost has moved further away. A diesel doing 55mpg against a petrol doing 45mpg now spends about 10p a mile in fuel where the petrol spends about 10.7p - near parity, where two years ago the diesel was clearly ahead.
Where diesel still wins
- High annual mileage. The efficiency advantage compounds. Above roughly 15,000 miles a year, and especially on motorways, diesel usually comes out ahead.
- Towing and load carrying. Diesel torque means less effort and less consumption penalty under load.
- Long steady journeys. Diesel engines are at their best warm and at constant speed.
Where petrol wins
- Short urban journeys. Diesel particulate filters need sustained higher-speed running to regenerate. Repeated short trips cause blockages and expensive repairs.
- Low mileage. Below about 10,000 miles a year the efficiency saving rarely covers the higher purchase price and servicing.
- Urban charge zones. Older diesels face charges in London's ULEZ and other clean air zones. Check the TfL ULEZ standards for your vehicle.
Fuel is not the whole cost
Diesel cars typically cost more to buy, more to service, and have expensive components - DPF, EGR valve, turbo - that petrol engines either lack or use more lightly. Against that, diesels have historically held value better on higher-mileage vehicles, though clean air zones have eroded that.
A fuel-only comparison flatters diesel. Run the whole-life numbers before switching.
Whichever you drive, where you fill matters more
The 21.2p petrol-diesel gap is fixed by the market. The 8p to 12p gap between the cheapest and dearest station near you is not - that one is yours to take. Check live prices on findpetrol, compare the brands in your area, or use our fuel cost calculator to compare two specific cars.
Frequently asked questions
At what mileage does diesel become worthwhile?
As a rough guide, above 15,000 miles a year with a good proportion of longer journeys. Below 10,000 miles, or if most trips are short and urban, petrol is usually the better choice on both cost and reliability.
Why is diesel more expensive than petrol when it used to be cheaper?
Duty is identical, so the difference is wholesale. European diesel supply tightened considerably following the changes to Russian refined product imports, and the UK imports a large share of its diesel. Petrol supply has been comparatively comfortable.
Is a hybrid cheaper than both?
Usually on fuel, in urban and mixed driving. On sustained motorway running a hybrid's advantage narrows considerably, since the electric assistance contributes least at steady speed.
Does premium diesel improve economy?
Marginally at best, and rarely enough to cover the price difference. Premium diesel's detergent additives may benefit an engine with existing deposits, but it is not a reliable route to lower running costs.
Will the diesel premium narrow again?
We do not forecast. What the data shows is that the gap is not fixed - it has been under 8p within the last year and stands at 21.3p now - so a buying decision that hinges on today's spread should be stress-tested at both ends before it is made.
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