Due to their higher rates of depreciation, it has long been assumed that vehicles from volume manufacturers such as Ford, Vauxhall and Renault are more expensive to own than similarly specified prestige models such as BMW, Audi and Mercedes. But new research from Glass's Market Intelligence Service looking at the whole life costs of diesel cars challenges this theory.

Glass's has analysed every aspect of the whole life cost for six upper-medium (family saloon/hatchback) diesel models over a period of three years and 60,000 miles. The selected cars, which have a comparable specification and power output, are: Audi A4 1.9TDi SE saloon, BMW 320d saloon, Ford Mondeo 2.0TDCi 130 Ghia hatchback, Mercedes-Benz C200 CDI Classic saloon, Renault Laguna 2.2 dCi Privilege hatchback, and Vauxhall [Opel] Vectra 2.0DTi Elite hatchback.

The research took into account the original purchase price (including the discount that a typical UK buyer might be expected to secure), depreciation (after three years and 60,000 miles), maintenance and repair costs, cost for replacing five tyres, road tax, insurance, interest on the capital (at 4%) and fuel consumption (using manufacturers' data).

Based on an analysis of this information, the BMW 320d has the lowest whole life cost, followed very closely by the Ford Mondeo TDCi. However, the BMW's position comes as a direct result of the free servicing package currently offered with the car - without it the Ford would be placed first.

Ranked third and fourth are the Audi A4 TDi SE and the Renault Laguna 2.2dCi Privilege. Of the six vehicles assessed, the most expensive models to own and run are the Mercedes C200 TDI Classic in fifth place, and the Vectra 2.0DTi Elite placed last.

"Particularly interesting is the fact that the difference in cost between the best and the worst performer is a mere £789 over the three years and 60,000 miles," said Alan Cole, consultant for Glass's Market Intelligence Service. "Depreciation remains one of the biggest factors influencing overall whole life costs, but buyers must always balance the strong residual values of prestige-brand cars against the higher purchase price and greater insurance costs."