How to choose between petrol and diesel

Cars

How to choose between petrol and diesel

by Adrian Porter

Does buying diesel really save you money? Here's our guide to choosing the right fuel type, plus our handy petrol vs diesel fuel cost calculator.

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When buying a new car, one of the first choices you'll make is whether you want a petrol or diesel engine. Most cars offer both options, and some brands also offer petrol- or diesel-electric hybrids.

Diesels have long been considered the best choice for frugal motorists due to 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 really the answer to cheaper motoring?

To find out, we’ve compared popular models to find out whether diesel really is the best choice when opting for a new car. You can also find our handy fuel cost calculator further down the page.

We publish realistic fuel economy figures for every car we test, and ignore the manufacturers’ claims. Discover the cars we recommend by viewing our Best Buys.

Petrol or diesel - which is cheapest?

To work out which is the most economical option, first look at the cost difference between the petrol and diesel versions of that car. Most manufacturers still ask a premium for diesel, typically £1,000-£2,000 more for models of the same specification. The question is: will you get that money back if you pay for diesel?

To work that out, you'll want to consider the running costs of each car, looking at fuel economy, fuel costs and car tax (VED).

Fuel economy: Diesel cars tend to get a better fuel economy, but can run into issues if you're not often running the car at high speeds (see diesel particulate filter, below). Naturally, the more miles you do, the more you can save from the better fuel economy. But with the average UK motorist now covering less than 9,000 miles per year, the potential fuel economy savings aren’t that impressive.

8,707The average mileage as revealed by the annual Which? Car Survey.

Fuel costs: Diesel fuel costs were higher than unleaded petrol by a few pence for many years. More recently, they have dropped to virtually the same price or occasionally a little cheaper than petrol, but they are now broadly the same.

Car tax: If bought before April 2017, car tax for diesel vehicles is usually cheaper than comparable petrols (for more information, see our guide to car tax explained).

Taking those three factors into account, you can then start to try work out if you're likely to get your money back. As an example, we've taken three sets of popular cars sold here in the UK, and compared the petrol and diesel variants.

For each pair of models, we've included the purchase cost, claimed mpg  with fuel cost for 10,000 miles, car tax and then shown the total one- and three-year cost, so you can see how they stack up.

Petrol vs diesel
Petrol vs Diesel - premiums and running costs
Fuel type and carPurchase cost (and car tax)

Claimed mpg (and 10,000 miles fuel cost)

Total one year costTotal three year cost
Petrol Ford Focus
(1.6 125 Style Powershift)

£18,845
F - £145)

44.8 mpg
(£1,128)

£20,118£22,665
Diesel Ford Focus
(1.5 TDCi 120 Style Powershift)

£19,740
(B - £20)

74.3 mpg
(£675)

£20,435£21,826
Petrol Vauxhall Corsa
(1.2 SE)

£12,745
(D - £110)

53.3 mpg
(£948)

£13,803£15,920
Diesel Vauxhall Corsa
(1.3 CDTi EcoFlex SE)

£14,995
(A - £0)

76.3 mpg
£657)

£15,652£16,968
Petrol Nissan Qashqai
(1.2 DiG-T Visia)

£18,545
(D - £110)

50.4 mpg
(£1,003)

£19,658£21,884
Diesel Nissan Qashqai
(1.5 dCi Visia)

£20,295
(A - £0)

74.3 mpg
(£675)

£20,970£22,321
Table notes
Three year costbased on 10,000 miles per annum, plus VED band tax for three years and initial car cost. Excludes maintenance costs.

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 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.

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. Though that may change in future years in the wake of emission scandals like Volkswagen Group's 'Dieselgate'.

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. However, rules will change in April 2017, with new cars bought after that date having to pay a flat rate of car 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.

Diesel particulate filter (DPF)

Our annual survey data shows that diesel-powered cars are also 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.

One unique issue for diesel cars is the diesel particulate filter (DPF), which can get clogged. The filter cuts down on harmful particulate emissions from diesel engines being released into the air, 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 keep the filter clear to avoid this. If your DPF does need replacement, the cost can run into thousands of pounds.

Fuel cost calculator: is the premium worth it?

Use our simple fuel-cost calculator to easily work out the fuel costs between two different cars. Enter two cars, their respective fuel economy figures (use the realistic mpg data from the ‘Tech specs’ part of each car review if possible, rather than the claimed figures), enter your mileage and see what the difference is. Current fuel prices can be looked up on sites such as the AA (theaa.com/motoring_advice/fuel) or PetrolPrices.com.