Diesel vs petrol running costs
Published - 5 min read
How to compare the real running costs of a diesel and a petrol car - fuel economy, pump prices, driving patterns, tax and clean air zones.
The trade-off in a nutshell
Diesel cars usually travel further on a litre of fuel than equivalent petrol cars, but diesel usually costs more per litre at UK pumps and diesel cars often cost more to buy. Which works out cheaper depends mainly on how many miles you drive and what kind of journeys you make.
Comparing fuel costs fairly
The fair comparison is cost per mile, not price per litre. Our MPG guide gives the formula. A useful shortcut:
- Divide the diesel car's MPG by the petrol car's MPG.
- If diesel's price per litre is higher than petrol's by a smaller proportion than that, the diesel is cheaper to fuel.
For example, if a diesel car does 55 mpg and the petrol version does 45 mpg, the diesel goes about 22% further per litre (55 / 45 = 1.22). So the diesel is cheaper to fuel unless diesel costs more than about 22% more per litre than petrol.
Over 10,000 miles, those two cars would use roughly:
| Car | MPG | Litres per 10,000 miles |
|---|---|---|
| Petrol | 45 | 1,010 |
| Diesel | 55 | 827 |
Multiply those litres by today's average prices on the UK Fuel Price Index to see the current difference in pounds. Use your own real-world MPG figures for the most accurate answer.
Your driving pattern matters
- Diesel tends to suit higher mileages and regular longer journeys, such as motorway commuting.
- Petrol tends to suit lower mileages and mostly short, urban trips.
Modern diesel cars have a diesel particulate filter (DPF) that traps soot and needs to burn it off periodically, which usually happens during sustained driving at higher speeds. A car used mainly for short trips may not get the chance, which can lead to warning lights and repair bills.
Other running costs
- Purchase price. Diesel versions of a model often cost more to buy than the petrol equivalent.
- Vehicle tax. Some diesel cars pay a higher first-year rate of vehicle tax than a petrol car with the same CO2 emissions. Check current rates on GOV.UK.
- AdBlue. Many modern diesels use AdBlue, a fluid that reduces exhaust emissions and needs topping up periodically.
- Servicing and repairs. Diesel emissions equipment such as the DPF can add to maintenance costs, particularly on high-mileage cars.
Clean air zones
London's Ultra Low Emission Zone and clean air zones in other UK cities set emissions standards that are generally stricter for diesel cars than for petrol cars, so an older diesel is more likely to face a daily charge than a petrol car of the same age. Check a specific vehicle with GOV.UK's clean air zone checker or Transport for London.
Bottom line
Work out cost per mile for the cars you're comparing, add the other running costs, and think honestly about the journeys you do most. For many lower-mileage drivers a petrol car (or a hybrid or electric car) is the better fit; for high-mileage motorway drivers, diesel can still make sense.