02 September 2011 by Lewis Kingston

  • Can a used diesel car save you money in the long run?
  • We show you how it could end up costing a fortune
  • Use our Cost of Motoring tool to compare running costs

If you want to save money on motoring, most people have one answer - buy a diesel.

The reality is, however, that a diesel car may actually end up costing you substantially more to run than a petrol-engined equivalent.

For a diesel to deliver worthwhile savings, thanks to its lower fuel consumption, you'll typically have to be driving more than 12,000 miles a year - preferably mostly on the motorway, where diesel economy is most beneficial.

A petrol car, that returns an average 30mpg, will cost £2,452 to fuel each year - based on 12,000 miles and an unleaded price of 135p a litre. A diesel car, returning a 45mpg average, will cost you £1,683 - assuming a diesel price of 139p a litre. 

That translates to a diesel saving of £769 a year, or £64 a month. Cut the diesel's economy to 40mpg - or increase the economy of the petrol car up to 35mpg - and that gap narrows further. Some may even be inclined to pay the premium to run a petrol car as they tend to be faster, quieter and more engaging to drive.

So, in that instance you're saving almost £800 a year by running a diesel car - right? Well, if everything works flawlessly then yes, you could. There is a chance, however, that it won't.

That's because diesel cars be more prone to issues, and more expensive to repair, than the equivalent petrol models. This is due to their complicated high-pressure fuel pumps, injectors that need coding to match the car, expensive turbochargers and fault-prone emissions control systems such as particulate filters and exhaust gas recirculation systems.

The latest data from Autoprotect, a leading aftermarket warranty provider working in partnership with Parkers, showed that the average cost of repair for a turbocharger system was £377, an injection system fix £302, a new fuel pump £341 and exhaust system repairs £389.

That means that a few failures - not unlikely on a used diesel car with some miles on it - can eliminate any savings you've made through cheaper fuel bills.

Many diesel cars - such as Ford's Mondeo - also have dual mass flywheels. These are designed to reduce vibration and can cost up to £2,000 to replace, and the clutch will typically need to be done at the same time. All of a sudden, the efficient diesel has ended up costing you over £1,000 more to run for a year than the petrol.

You can replace many dual mass flywheels with a single mass flywheel that will not fail in the same way - although you may notice a slight increase in noise and vibration. Some petrol cars have dual-mass flywheels too, yet they're less prone to failure due to reduced levels of torque and vibration and consequently not a major concern.

So, if you're considering buying a used diesel car then tread carefully, do the sums and bear the aforementioned points in mind - especially if your annual mileage is low.

Note: Prices correct at time of publication.

Parker's Top Tip

You can compare both new and used car running costs by using our Cost of Motoring tool. If you're concerned about the reliability of a used car, you can always buy a second-hand car warranty to protect your wallet from the worst.