Buying a car Petrol vs diesel buying guide

Does buying diesel really save you money? Or is petrol the better buy? Here's our guide to choosing the right engine for your car, plus our handy petrol vs diesel fuel cost calculator.

Which should you decide, petrol or diesel?

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.

Petrol or diesel - which is cheapest?

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 typeCar and enginePurchase cost

Claimed mpg

(and fuel cost after 10,000 miles)

Road taxTotal one year costTotal three year cost
PetrolFord Focus - 1.6 125 Style Powershift£18,845

44.8 mpg

(£1,128)

F - £145£20,118£22,665
DieselFord Focus - 1.5 TDCi 120 Style Powershift£19,740

74.3 mpg

(£675)

B - £20£20,435£21,826
PetrolVauxhall Corsa - 1.2 SE£12,745

53.3 mpg

(£948)

D - £110£13,803£15,920
DieselVauxhall Corsa - 1.3 CDTi EcoFlex SE£14,995

76.3 mpg

(£657)

A - £0£15,652£16,968
PetrolNissan Qashqai - 1.2 DiG-T Visia£18,545

50.4 mpg

(£1,003)

D - £110£19,658£21,884
DieselNissan Qashqai - 1.5 dCi Visia£20,295

74.3 mpg

(£675)

A - £0£20,970£22,321

Table notes

  1. Three year cost based on 10,000 miles per annum, plus VED band tax for three years and initial car cost. Excludes maintenance costs.
  2. Petrol cost for calculation: 111.2pence per litre. Diesel cost for calculation: 110.4 pence per litre. Prices as of October 2015.
  3. Petrol/diesel variants selected to be as identical as possible, primarily in terms of BHP, and of the same trim level (for comparable purchase cost).

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.

Choosing between petrol and diesel – other considerations

Residual values of petrol and diesel cars

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.

Car tax and insurance

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’.

Insurance and servicing costs

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.

Other running costs

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.

Petrol vs diesel fuel cost calculator: is the premium worth it?

 

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Example running costs comparing a petrol with a diesel car
Example numberCar price differenceAnnual fuel cost petrolAnnual fuel cost dieselAnnual CO2 emissions petrolAnnual CO2 emissions dieselNo. of years to recoup diesel price premium
1£1,500£1,068£8522.442.216.9
2£2,000£1,330£9913.042.575.8
3£1,500£1,575£1,1713.63.043.7
4£1,850£1,991£1,3534.553.522.8
5£1,099£1,805£1,4724.123.833.3

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.







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Other sections in this guide

  1. Overview
  2. Petrol vs diesel buying guide