Manheim Auctions claims fleets that are quick to react to changing market conditions avoid losing their companies thousands of pounds in lost disposals revenue.
Companies with the most to lose are high mileage fleets that keep petrol cars longer than 70,000 miles and diesel cars into six figures.
Martin Potter, corporate sales director, says keeping cars for too long in the current market conditions is a false economy, even when running high-tech diesels.
He said: 'The market has changed considerably over the past couple of years in that there are more cars coming back into the market at three years old. Consequently, buyers have a large choice of cars to bid for, and those that aren't in A1 condition, or with the wrong mileage, are ignored.
'The result is that any petrol car with over 70,000 miles is proving tough to sell, while diesels with more than 80,000 miles are likely to be more difficult to shift.' Manheim claims fleet managers will need to think long and hard over diesel replacement cycles.
It suggests that diesel cars should be run up to a maximum of 80,000 miles over four years, with 70,000 miles the maximum for petrol cars over the same period.
2yrs/up to 40,000 miles
3yrs/up to 60,000 miles
4yrs/up to 70,000 miles
2yrs/up to 60,000 miles
3yrs/up to 80,000 miles
4yrs/up to 80,000 miles