Orders of medium-duty vehicles in Classes 5-7 increased 4% to 226,800 in 2015, and truck manufacturers built 237,100 units, according to ACT Research.

Orders closed out the year on a high note, said Steve Tam, ACT's commercial vehicle sector vice president.

The Class 5 market has remained in growth mode since reaching a low in early 2009 at the depth of the recession, and has outstripped growth of classes 6 and 7, said Tam.

"Class 5 build and sales were the highlights of 2015, setting a record for the best year ever," Tam said. "Classes 6-7, on the other hand, failed to register in the top 10, with 2015 arriving in 13th place."

Since the rebirth of the Class 5 segment in the 1980s, truck fleets have gained a greater sensitivity to total cost of ownership and have made a subtle shift down from Classes 6-7, Tam said.

"Conversely, the Classes 6-7 market has struggled, largely because of its exposure to the non-residential construction and municipal markets," Tam said. "While they have improved and are expected to continue doing so, they have lagged behind the broader market."