The average retail selling price for a used Class 8 truck dropped to $48,643 in May, down 6% month over month but 3% above the average price in May 2011. On a year-to-date basis, prices remained 13% above last year's pace, according to ACT Research Co.

's State of the Industry: U.S. Classes 3-8 Used Trucks.

"Even though prices are showing volatility on a month to month basis, they continue to show solid growth for longer-term comparisons," says Steve Tam, vice president, commercial vehicle sector with ACT. "We expect that pattern to be the norm though 2012."

The report from ACT provides data on the average used price for the top-selling Class 8 model for each of the major truck OEM's - Freightliner (Daimler); Kenworth and Peterbilt (Paccar); International (Navistar); and Volvo and Mack (Volvo).

ACT is a publisher of new and used commercial vehicle industry data, market analysis and forecasting services for the North American market, as well as the U.S. tractor-trailer market and the China CV market.