The findings come from the new Manheim Market Report, which tracks the last 30 days' worth of prices achieved on cars going through its 18-strong auction network.
It looked at average prices for the four key models in the upper-medium sector – Ford Mondeo, Vauxhall Vectra, Renault Laguna and Volkswagen Passat.
Taking the hundreds of 2000 X-registered models that have been sold by Manheim in the past 30 days with between 40,000 and 100,000 miles on the clock, on average, each car was sold with between 70,500 miles, which is proof of the trend that fleets are deciding to extend leasing contracts.
Each car has out-performed CAP clean predictions in the past 30 days and in the case of the Ford Mondeo by an average of 5% on each vehicle sold. A spokesman said: 'Across the four models, diesels have performed well, particularly the Mondeo and Vectra. On the petrol side, all four models have generated prices above CAP Clean.
'The range-topping V6 Ghia, GSi, 20v Sport and V6 versions of the respective Mondeo, Vectra, Passat and Laguna all generated lots of interest and made the strongest prices.
'Due to the high volume of upper-medium fleet cars going through auction, condition and mileage is increasingly important as the better the car, the higher the price and the quicker it will sell. This will be particularly relevant in 2004 when many of the extended contract cars start coming through with higher mileages on the clock.'
The company also stressed that for vehicles over three years of age a full service history was vital. With the standard manufacturer warranty expiring after three years, buyers will be faced with fixing any repair work themselves.
'The peace of mind that a full service history provides will add value to the vehicle,' the spokesman added.
Average prices against average CAP Clean value
X 00 Ford Mondeo 69,000 miles £3,995 (105% CAP Clean)
X 00 Vauxhall Vectra 70,000 miles £4,277 (104%)
X 00 Renault Laguna 69,000 miles £3,785 (101%)
X 00 VW Passat 72,000 miles £5,735 (101%)