Petrol vs Diesel: which is right for you?
Diesel costs more per litre but goes further per tank. Petrol is cheaper to buy but burns through more fuel on long runs. The right choice depends almost entirely on how far you drive each year, and increasingly, where you drive.
The core difference in fuel economy
Diesel engines extract more energy from each litre of fuel, typically delivering 20–30% better fuel economy than a comparable petrol engine. On motorway runs, this gap is most pronounced. In stop-start city driving, the advantage shrinks considerably. A diesel car doing 55 mpg on the motorway might only manage 42 mpg in heavy urban traffic, while its petrol equivalent drops from 42 mpg to 36 mpg.
The efficiency advantage exists because diesel fuel contains about 15% more energy per litre than petrol. Diesel engines also run at higher compression ratios, which extracts more work from each combustion cycle.
The cost per mile calculation
Despite better fuel economy, diesel isn't automatically cheaper to run, because diesel costs significantly more per litre than petrol. At typical April 2026 prices (petrol ~158p, diesel ~191p), the per-mile cost works out roughly as follows for a medium-sized family car:
At current prices, diesel is actually more expensive per mile in city driving due to the large price gap between fuels. On motorways and longer runs, diesel's superior fuel economy pushes the cost per mile below petrol. If your driving is mostly urban, petrol now wins on running costs. Diesel only becomes cheaper per mile once you're doing significant motorway mileage.
The break-even point
Diesel cars typically cost £1,500–3,000 more to buy than their petrol equivalents. They also tend to have higher servicing costs. Against that, the fuel saving at 15,000 miles per year might be £250–400 annually. That puts the break-even point at 4–8 years of ownership, which is why diesel makes most sense for high-mileage drivers who keep a car for the long term.
Diesel cars fitted with a Diesel Particulate Filter (DPF) need regular motorway runs to burn off accumulated soot. If you only do short urban journeys, the DPF can block, requiring an expensive regeneration or replacement. For drivers doing under 10,000 miles a year on short runs, petrol is almost always the better choice.
Clean Air Zones and future costs
An increasingly important factor is the spread of Clean Air Zones (CAZs) in UK cities. Older diesel cars (pre-Euro 6, typically pre-2015) face daily charges in cities including Birmingham, Bristol, Bath, Bradford and Portsmouth, typically £8–£9 per day. Older petrol cars (pre-Euro 4, pre-2006) are also affected, but the diesel restrictions are broader. If you regularly drive into CAZ cities, this can add thousands per year to the cost of running a non-compliant diesel.
| Factor | Petrol | Diesel |
|---|---|---|
| Fuel economy | 35–45 mpg | 45–58 mpg |
| Fuel cost per litre | Lower (~133p) | Higher (~138p) |
| Best for city driving | Yes | Acceptable |
| Best for motorway | Good | Better |
| Clean Air Zone risk | Lower | Higher (older models) |
| Purchase price | Lower | £1,500–3,000 more |
| Short-run driving | Fine | DPF risk |
| Break-even mileage | — | ~12,000+ miles/year |
Under 10,000 miles/year or mostly urban: petrol. Over 15,000 miles/year with regular motorway use: diesel probably saves money. In between: petrol is safer and simpler.