May showed a continued decline in the availability of low-mileage used heavy-duty trucks, according to analysts with Great American Group LLC.

According to the American Truck Dealers and Used Truck associations, the average mileage for May was 560,000 miles, which is a historic high compared to a year ago, when the average mileage was 514,000 miles.

Dealers also noted a shortage of lower-priced trucks in the $25,000 to $40,000 range.

"Fleet managers are keeping trucks longer and prices for new trucks selling at $120,000 or more have slowed in 2012," says to Michael A. Petruski, executive vice president and general manager of Great American Group's Machinery & Equipment Division. "The average sales price for a used truck in May was $41,000, down from April, but 2% higher than in May 2011. The average retail price for a sleeper was $48,800 in April 2012, compared to $44,100 in April 2011."