Registrations of used commercial vehicles rose 7.8% in the first nine months of 2013 over the same period last year, according to auto data firm R.L. Polk & Co.

About 527,500 used commercial vehicles were registered in that timeframe, the second-highest nine-month span since 2004, Polk said.

During the same period, new commercial vehicle registrations rose only 2.8% over 2012, Polk said in a Dec. 12 report.

"The continued strength of used commercial vehicle transactions during the first three quarters of the calendar year reflects a competitive market for clean used equipment as it becomes available," said Gary Meteer, Polk's director of aftermarket and commercial vehicle solutions.

The total commercial vehicle population was relatively flat in the first nine months as fleets focused on replacing old equipment.