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Petrol vs Diesel Cars in 2018: Which is Better?

By Adrian Porter

Is it better to buy a diesel or petrol car in 2018? Our expert guide reveals whether diesel really is the cheaper, wiser choice.

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Diesel cars have long been favoured by the frugal motorist, thanks to superior fuel economy and less tax. But with new diesel cars no longer benefiting from low car tax, residual values set to decrease and concerns over air pollution from diesel cars driving new legislation, is it still the fuel of choice?

Manufacturers charge a premium for diesel models compared with comparable petrol cars. But with petrol engines getting ever more economical, manufacturers of diesel cars may no longer be able to justify their higher prices.

Which? ignores manufacturers’ fuel-economy claims and publishes more realistic mpg figures from our own tests. Discover the cars we recommend by viewing our round-up of the best cars.

Diesel cars vs petrol: in a nutshell

  • Many modern diesel cars are fitted with a diesel particulate filter (DPF), which can clog if you don't often drive at motorway speeds. It's a pricey fix, so best avoid diesels if you avoid motorways.
  • Diesel cars tend to cost more to buy than an equivalent petrol car. In the past, it was possible to make this back thanks to diesel cars having lower car tax and fuel costs.
  • Since the new car-tax change, from the second year of ownership you'll pay a standard rate of £140 a year. This means it will take you longer to recoup the higher on-the-road price you pay for a diesel car, as you will now only save between £105 to £125 on running costs per year (largely thanks to better fuel economy).
  • Diesel cars also tend to be the fuel of choice for those who need to tow thanks to diesel engines producing huge amounts of torque (pulling power) - but petrol and hybrid alternatives are available. Here's our guide to buying the best tow car.

Still not sure if you should choose petrol or diesel? Keep reading to find out.

Diesel vs petrol fuel economy 

Want to know which cars are the most efficient? See our round-up of the most fuel efficient cars.

Diesel cars tend to have better fuel economy. So, despite diesel fuel typically costing slightly more than petrol (by 1p to 2p a litre), a diesel car's fuel costs will be cheaper than the equivalent petrol car. 

Our unique Which? tests show that, on average, diesel cars are more efficient by about 8mpg (based on tests of 249 cars that all meet the latest emission limits: Euro 6). That doesn't sound like much, but can save you around £200 a year. 

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

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 a year, the potential fuel-economy savings aren’t that impressive.

Fuel-cost calculator: how much will you pay?

You can use our simple fuel-cost calculator to easily work out the fuel costs between two different cars. Enter the respective fuel-economy figures for two cars and your mileage to see the difference. 

For the most accurate results, don't use the manufacturers' claimed figures. Instead, use our realistic mpg data from the ‘Tech specs’ part of our independent, expert car reviews.

Current fuel prices can be looked up on sites such as TheAA.com or PetrolPrices.com.

Diesel vs petrol car tax 

How much car tax you pay depends on when you bought your new car. 

  • Before April 2017: diesel cars are usually cheaper than comparable petrol cars. 
  • 1 April 2017 onwards: only the first year's payment is based on CO2 emissions (diesel cars have lower levels of CO2 to comparable petrol cars). From the second year of ownership onwards, owners pay a standard rate of £140 per year.

This means a large portion of the savings offered by more frugal diesel engines have now been swallowed by the change in car-tax rates.

Under the old system, if you owned a diesel Ford Focus you would pay nothing in car tax for the first year. Then just £20 a year. If your car's CO2 levels were under 100g/km, like the diesel Nissan Quashqai, you would have been exempt from car tax completely. 

Now owners of the same cars have to pay between £120 and £140 in the first year, and £140 per year afterwards. That's an increase of £120-£140 per year over the older rules.

For more information on how car tax has changed, see our guide to car tax explained.

Diesel vs petrol: which is cheapest?

Taking purchase cost, first year tax rate and fuel economy into account, it's possible to work out which fuel version will be better in the long run. 

To show you, we've taken a petrol and diesel version of the three of the most popular cars sold here in the UK: the Ford Focus, Vauxhall Corsa and Nissan Qashqai

Before the 2017 April car tax change, the diesel version of the Ford Focus would have worked out cheaper than the petrol within five years. Now, not only does it take years longer, but the amount saved once the diesel version becomes cheaper has significantly lessened. 

Our table, below, shows you the full data behind the graphic, and additional cars:

Petrol vs diesel
Petrol vs diesel: premiums and running costs
Fuel type and car Price as new

Claimed mpg (and 9,000 miles fuel cost)

Total one-year cost Total five-year cost
Petrol Ford Focus
(1.5T EcoBoost 150PS)

Includes first year car tax (£160)

51.4 mpg

£22,962 £27,150
Diesel Ford Focus
(2.0 TDCi 150PS)

Includes first year car tax (£140)

70.6 mpg

£24,243 £27,475
Petrol Vauxhall Corsa
(1.4i, 75PS ecoFLEX)

Includes first year car tax (£160)

54.3 mpg

£13,819 £17,815
Diesel Vauxhall Corsa
(1.3 CDTi EcoFlex SE)

Includes first year car tax (£140)

72.4 mpg

£15,742 £18,910
Petrol Nissan Qashqai
(1.2 DIG-T 115)

Includes first year car tax (£160)

50.4 mpg

£22,020 £26,280
Diesel Nissan Qashqai
(1.5 dCi 110)

Includes first year car tax (£120)

74.3 mpg

£23,440 £26,540

Table notes

Price as new is the manufacturers OTR (On The Road) price, and includes the first year of car tax, shown separately. Five year cost based on driving 9,000 miles per annum, the standard car tax rate of £140 per year, plus the initial car cost. Excludes maintenance costs. All cars have a manual transmission, have a five-door body and are all front-wheel drive. Claimed fuel economy is used as we do not have Which? tested figures for all of the specific engine variants in the table. Prices correct as of April 2017.

How long do I need to own a diesel car for it to work out cheaper than petrol?

Before the 2017 April car-tax change, the diesel version Ford Focus would have worked out cheaper than the petrol after just three years. Now, not one diesel version of the above car becomes cheaper within five the first years of ownership:

  • The diesel Nissan Qashqai becomes cheaper than the petrol after six years
  • The diesel Ford Focus becomes cheaper than the petrol after seven years
  • The diesel Vauxhall Corsa becomes cheaper than the petrol after 11 years
£105-£125How much you save on running costs per year if you buy diesel over petrol

The fuel savings offered by diesel over petrol, as shown by the cars above, averages out at £245 a year. But with £120-£140 of that saving now going on car tax, you're only really saving £105-£125 by opting for diesel. 

That means it'll take a much longer time to earn back that premium you paid for a diesel engine.

Diesel cars and residual values 

In previous years, diesel cars generally retained their value better than petrol versions. This was thanks to better fuel economy and lower car tax rates. 

But since 1 April 2017, diesel cars no longer benefit from lower car tax. Plus some cities are introducing new charges that penalise older diesel cars. So diesel cars are now less likely to hold their value. 

Diesel particulate filter (DPF)

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.

Our annual survey data shows that diesel-powered cars are slightly less reliable than petrol ones. While routine maintenance costs are similar for petrol and diesel, it is potentially more expensive to repair a diesel if anything serious goes wrong.

Want to know which cars you can trust? We reveal the most reliable car brands.

Will diesel cars be banned?

Yes, the sale of new diesel and petrol cars will be banned in 2040. This was announced as part of the UK Air Quality plan published on the 26 July 2017.

The government’s plan requires local councils to reduce vehicle emissions in their area, using a range of measures such as:

  • Changing road layouts at congestion and air pollution pinch points
  • Encouraging people to buy electric vehicles
  • Retro-fitting buses to give lower emissions
  • Investing in new low-emission buses
  • Encouraging the use of public transport.

If these measures fail, local authorities could introduce restrictions, such as charging zones, or stopping certain cars from using designated roads at set times.

However, the plan also states that these restrictions should be removed in the event that emission levels fall enough to be legally compliant and ‘there is no risk of legal limits being breached again’.

A £255m fund has been set up to help local councils execute their emissions-reduction plans, with £40m being made available immediately.

Several cities around the world have committed to banning diesel cars by 2025.

Emissions and air pollution

Diesel cars produce a lot more NO2 (nitrogen dioxide) than petrol cars. It is thought that nitrogen dioxide inflames peoples' airways and has been linked to increased hospital admissions and premature death.

To find out more about emissions and to see which carmakers produce the dirtiest diesel cars, see our guide to air pollution from cars.

Older cars have to meet EU emission levels under a standard called Euro 5. Newer cars have to meet a stricter level: Euro 6. 

But our more realistic Which? tests have discovered that for years diesel cars have been emitting a lot more NOx than allowed by the limits of the official tests.

London T-Charge and Ultra Low Emission Zone

From October 2017, the T-Charge will take effect in London. This means you will have to pay up to £22.50 per day to drive an older car in London.

Any car – petrol, diesel, hybrid or otherwise – that does not meet Euro 4 emission laws (introduced Jan 2005, mandatory for all new cars as of Jan 2006, though some may have complied earlier than this) will have to pay an extra £10 per day on top of the regular charge (£11.50) to come into the congestion charge zone. 

The T-Charge ends in April 2019, when the Ultra Low Emissions Zone (ULEZ) comes into force. Owners of petrol cars that don't meet Euro 4 or better will have to pay £12.50 on top of the congestion charge (£11.50). Drivers of old petrol cars will pay £24 to enter the ULEZ.

Diesel cars will have to meet Euro 6 (introduced September 2015, mandatory for all new cars as of September 2016, though some cars will have been Euro 6 compliant earlier) to escape this extra charge. By 2019, diesel cars over four years may be liable to pay £24 to enter the ULEZ.


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