Used car buyers tempted by frugal diesels could have to do up to 56,373 miles before reaping the financial benefit of the fuel savings they believe are on offer, according to used car price guide WiseBuyer's.

On average, the cost of buying a used diesel is between 8-12% higher than its petrol equivalent and service costs for some diesel models are also higher. As a result owners need to complete thousands of miles before they break even.

WiseBuyer's data shows that a year-old Ford Focus 1.8 TDCi LX typically costs £1,150 more on the forecourt than its 1.6 LX petrol alternative. Although the former records an average 53.3mpg compared to 40.9mpg for the petrol version, the actual fuel cost saving is only 2.04 pence per mile travelled. So owners would need to complete 56,373 miles before fuel savings recoup the extra cost of buying a diesel.

Even a small city car, the Volkswagen Lupo 1.4 TDi PD Sport, which costs just £350 more than its 1.4 Sport petrol cousin, needs 14,286 miles to be covered before the owner starts benefiting from the more economical diesel engine.

Purchasing a seven-seater Ford Galaxy 1.9 TDi Zetec diesel would demand the owner completing 43,237 miles before matching the £1,950 cheaper 2.3 Zetec petrol equivalent.

Nic Barfield, editor of WiseBuyer's, said that too few motorists hunting for a used car take into consideration the true cost of buying a diesel.

"For someone who only does 10,000 miles a year, a diesel may not be the best option. As the research shows, it could take up to five years of average motoring to start feeling the financial benefit of the fuel savings."

Diesel sales have accelerated over the past five years, from 14.1% market share in 2000 to an estimated 33% in 2004, levels similar to those in the rest of Europe.

"However, diesel fuel on the Continent is significantly cheaper, meaning that car price differences are eroded very quickly," Barfield noted.

He added: "Manufacturers have done a fantastic job in advancing diesel technology and improving its appeal. Unfortunately, a great deal of their good work is undone by the tax policy of the British government."