Other sections in this guide
- Petrol vs diesel buying guide
Find out how frugal cars must be to pay back that diesel premium
Choosing between a petrol or a diesel engine is one of the first considerations you’ll make when buying a new car.
Diesels have long been considered the obvious choice for frugal motorists due to a superior fuel economy and lower car tax.
But with diesel emissions being scrutinised, manufacturers demanding a premium for diesel models and petrol engines getting ever more economical, is diesel still the answer to cheaper motoring?
We’ve compared popular models to find out whether diesel really is the best choice when opting for a new car.
For any car you're considering, find out how the diesel and petrol costs stack up by using our handy interactive tool, below.
Find out real-world fuel economy figures - sign up to Which? for £1 to gain access to all our car reviews.
Most manufacturers still ask a premium for diesel – typically £1,000-£2,000 more for models of the same specification. However, road tax for diesel cars is usually cheaper than comparable petrols, and diesel fuel costs have been comparable, if not occasionally cheaper, than petrol in recent times. So it's not obvious as to which might be cheaper in the long run.
To help make sense of it all, we've compared the purchase and running costs for three sets of popular cars sold here in the UK, looking at both the petrol and diesel variants. For each set of models, we've included the on-the-road purchase cost, claimed mpg with fuel cost for 10,000 miles, road tax and then show the total one and three year cost, so you can see how costs compare between the two types of engine.
|Petrol vs Diesel - premiums and running costs|
|Fuel type||Car and engine||Purchase cost|
(and fuel cost after 10,000 miles)
|Road tax||Total one year cost||Total three year cost|
|Petrol||Ford Focus - 1.6 125 Style Powershift||£18,845|
|F - £145||£20,118||£22,665|
|Diesel||Ford Focus - 1.5 TDCi 120 Style Powershift||£19,740|
|B - £20||£20,435||£21,826|
|Petrol||Vauxhall Corsa - 1.2 SE||£12,745|
|D - £110||£13,803||£15,920|
|Diesel||Vauxhall Corsa - 1.3 CDTi EcoFlex SE||£14,995|
|A - £0||£15,652||£16,968|
|Petrol||Nissan Qashqai - 1.2 DiG-T Visia||£18,545|
|D - £110||£19,658||£21,884|
|Diesel||Nissan Qashqai - 1.5 dCi Visia||£20,295|
|A - £0||£20,970||£22,321|
In the table above, the diesel version of Ford Focus does comes up cheaper after three years, but the diesel version of the Vauxhall and Nissan cars are still dearer. In fact, you'd have to run the Vauxhall for nearly eight years before you started to see a return on your money compared to the petrol version - and that's without any maintenance costs included.
If you're currently weighing up between petrol and diesel, have a look at our fuel cost calculator at the bottom of this page. Though it does not include road tax, it will give you an idea of how many years it will take for the diesel option of a car you're considering to come up cheaper compared to a petrol equivalent.
The above table does, however, take the fuel economy (mpg) at face value. We know that cars almost never achieve their claimed fuel economy. If you want to find out how fuel efficient your car really is, use our mpg calculator, or find your car in our independent car reviews, where we publish a more realistic mpg for every model and engine we test.
Diesel cars generally retain their value better than petrol versions. They’re currently in high demand, thanks to people looking for cars with better fuel economy and lower car tax rates.
However, our latest survey data shows that diesel-powered cars are slightly less reliable than petrol ones. And, while routine maintenance costs are similar for petrol and diesel, it is potentially more expensive to repair a diesel if anything serious goes wrong. In particular, diesel engines use particulate filters, which can get clogged (especially if only used for short journeys), and the cost of replacement can stretch into thousands of pounds.
Diesel engines are inherently more efficient than their petrol opposites, so the CO2 levels are usually lower, resulting in correspondingly lower car tax liability. However, as the diesel emissions scandal has shown, diesel engines can be polluting when it comes to nitrogen oxides – so much so that some local councils in the UK are beginning to introduce a so-called ‘diesel tax’.
There aren't usually big differences in servicing costs, but insurance costs can vary (they can favour petrol or diesel, so check before you buy), and the car tax is usually slightly cheaper for diesels. Factor this into your estimations, but remember that the differences will be small compared with the likely fuel savings.
One other issue to consider is whether the car is fitted with a diesel particulate filter (DPF). This cuts down on harmful particulate emissions from diesel engines, but there are many reported cases of these becoming clogged and needing replacement. Most owners' handbooks advise running the engine at high speed, for example on a motorway run, to clear the filter and avoid this. If your DPF does need replacement, the costs can run into thousands of pounds.
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|Example running costs comparing a petrol with a diesel car|
|Example number||Car price difference||Annual fuel cost petrol||Annual fuel cost diesel||Annual CO2 emissions petrol||Annual CO2 emissions diesel||No. of years to recoup diesel price premium|
If you buy diesel over petrol, just how long will it take to be back in the black?
The 'diesel payback' period could take much longer than you would think - especially for lower mileage drivers (the average Which? members does less than 9,000 miles per year).
The calculator on the right only takes initial purchase price, fuel costs and mileage into account, not road tax as in the table above, but should give you a rough idea of how many years’ driving it will take to make buying a diesel the most cost-effective choice.