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Which? Confirms Hidden Costs of Running Diesel Cars

Which? has confirmed Honest John's warnings here that the running costs of diesel cars can often outweigh their economy benefits, particularly after the cars pass three years old.

Despite superior fuel economy and usually lower car tax, a Which? investigation has found that new diesel cars are often more expensive to run than petrol cars.

This year is set to be the first in which diesel cars will make up more than half the new car market. However, a Which? comparison of diesel and petrol versions of six popular car models has found that even when the cars are new, petrol engines can be the more cost effective choice for drivers covering a typical annual mileage.

Diesel engines in the popular BMW 5 Series, Ford Fiesta, Vauxhall Astra, and Volkswagen Tiguan may deliver cheaper fuel bills than their petrol counterparts initially but it takes many years before they actually save the average driver1 money.

With drivers having to pay a premium for a diesel car, typically £1,000 to £2,000 more on a new car, Which? tests reveal that it could take up to 14 years to recoup the upfront costs in fuel savings. Lower pump prices for petrol and improvements in petrol engine economy mean that petrol cars now often provide better value for money.

The biggest mistake of all is to buy a high mileage 3-year-old ex-fleet diesel car on the false assumption that diesel engines are robust enough to take the mileage and continue to offer fuel economy advantages. All too often, dual mass flywheels,  diesel particulate filters, EGRs and turbos fail, potentially landing the car buyer with bills greater than the vale of the car. See FAQ.

Which?’s own fuel-economy tests also often fell short of the manufacturers’ claims for both diesel and petrol cars, meaning that motorists shouldn’t place their faith in official miles per gallon (mpg) figures.

The Which? study also considered reliability, taking information directly from the 2012 Which? Car Survey, which found that petrol cars are generally more reliable than diesels – both in the first three years of their life (the typical warranty period), and even more so between four and eight years-old.

Notes to editors

    1. Which? tested popular cars from superminis to estates and 4x4s, looking at identical-spec petrol and diesel versions of the Ford Fiesta, Vauxhall Astra, Volkswagen Tiguan, Volkswagen Sharan, BMW 5 Series, and Peugeot 308 SW. We calculated the annual fuel bill for each based on a mileage of 10,672 (the average annual mileage in the 2012 Which? Car Survey).
    2. In the year up to May 2012 diesel cars accounted for 51 per cent of new car sales, with petrol cars at 47.5 per cent. Alternative fuel cars accounted for 1.5 per cent.
    3. Which looked at two scenarios when testing the cars: Average combined use of town, country and motorway driving; and predominantly driving around town. But for Which car reviews, Which tests urban, extra urban, motorway (and all combined) on all the cars. Full details of the tests for 555 new and used models are available online at www.which.co.uk/cars Potential problem areas of 2,000 cars here: www.honestjohn.co.uk/carbycar 

The Which? Car Survey 2012 is featured in the August edition of Which? Magazine.


Brocksdown    on 19 July 2012

The tables in the Which? report didn't consider residual values at all.  Apart from the fuel savings, my '09 XF 3.0d cost £1k more than the petrol 3.0, and is now worth £4k more at trade-in. And it's a better drive!  A typical Which? headline grabber with little substance.  The real question is why diesel in the UK costs 15-20p more a litre than our European neighbours are paying, whereas petrol costs are about the same.

Stuart    on 19 July 2012


Honestjohn    on 20 July 2012

Residual values rely on the public who buy diesel cars after they come off the fleets at 3 years old not realising the costs they face. For example, one reader bought a 3 year old 80,000 mile Focus 1.6TDCI 110 at auction for about £3,500. Within four months she was faced with having to fit a new Dual Mass Flywheel, a new Diesel Particulate filter and a new regulator for the Eolys type DPF. That cost her more than £2,500. Turbo failures, turbo oil pipe failures and EGR failures are also common on diesels at this sort of mileage. As well as the need to replace timing belts. Petrol engine turbos and timing belts can also fail, of course. But diesels are more than twice as likely to need expensive replacements between years 3 and 6 than petrols are.

Diesel_Pig    on 20 July 2012

As usual the initial cost comparison is between full retail value of petrol car against diesel version, who pays full list for a car these days?

I purchased a new 1.6 TDCi Fiesta in Nov 2011, for the same money I was offered a high spec petrol version and the saving over list was nearly £3.5K with all the extras, according to the Which magazine driver tool my savings were instant and the fuel saving paid for the yearly service.

I'm fortunate in that my drive to work is motorway and the car is not used for short distance so I get the maximum benefit of fuel saving but when the new Fox engined Fiesta is available I shall switch back to petrol.

Agree with HJ's comments on residuals, I would never buy a high mileage diesel ( I know plenty of fools who have! ) I intend to trade my car while it has low miles and value.

skidpan    on 20 July 2012

Our BMW 118D cost £300 more than a 118i in March 2008 when we bought it new. We save over £100 in RFL thus in 4 years we have saved the initial cost difference. It does an average of 48mpg (calculated) wheras petrols we have been loaned when it goes in for service do about 34mpg (on the dash), at £6,00 a gallon over 10,000 miles a year that is a £500 saving at least. Servicing costs are identical. Over the 4 years we have owned it we have saved £2000. Add to that residuals, Parkers say the 118D is worth approx £900 more than the 118i thus our total saving so far is approx £2900.

We tried a 118i and a 118D before we ordered, the 118i was poor in comparison to the 118D.

In the years ahead we may have dpf issues but without a crystal ball who knows. It regens when needed, what more can we do. Both the petrol and diesel use chain cams and have dmf so no difference there.

The longer I keep the car the greater the savings, if I have to spend some of that on a repair I will just have to do it. At least its a better drive than the petrol version.

Honest John and Which both have a habit of looking at only a small part of the story. Having run diesels sing 1996 I know which I will continue to buy.

voy2age    on 20 July 2012

people must be getting diesel fuel cheap??...thats all i'm saying on the matter..:-) 

skidpan    on 20 July 2012


Don't understand your comment, I pay about £1.339 per litre for diesel from Tesco's, that is £6.09 per gallon which is why I used the £6.00 approximation in my calculation Petrol is £1.299 or £5.91 per gallon, very little difference. Some local independents charge virtually the same for both fuels, a Gulf station about 3 miles away was charging £1.469 for petrol and £1.479 for diesel when we passed last Saturday, now that is expensive.

If you are hinting that we use red diesel your post should be deleted.

voy2age    on 21 July 2012

cheap as in. fell off the back of a lorry   :-)

   on 10 August 2012

Been done a few times already, the problem isn't with the cars, more the people who buy them and don't use them as intended.  Diesel Performance Round Rock

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