The leading car auctioneer, BCA, has warned that buying a diesel car to save money in the long run might not always to be best option for private buyers.

It says that, although a diesel version of a car will sell for more than the equivalent petrol version at resale time (mostly because it would have been around £1,000 more to buy new), the petrol may actually lose a smaller proportion of its value.

Tony Gannon, BCA Communications Director commented, "There can be little argument that the average price of a diesel car always outperforms that of a similar average petrol model... However, when looking at the retained value over the typical 40 month working life of a volume fleet & lease car, the petrol car – perhaps surprisingly – is the stronger performer by some three percentage points."

That means that private buyers need to do some careful sums when choosing between petrol and diesel. The benefit of diesel depends on the mileage you do and the type of car you are buying. No-one is going to choose petrol over diesel for a big off-roader, but conversely, few people would benefit from buying a diesel version of a city car.

Broadly speaking, petrol usually makes more sense for cars smaller than a Focus or Golf, and diesel usually makes sense for cars larger than that (depending on individual circumstances). For the classic Focus-sized family hatchback, it is quite finely balanced and that is where the individual really needs to do some careful analysis.