North American Class 8 net orders, while below April's overheated intake, were still 77 percent about the same month last year, continuing to fuel growth in the industry backlog.

Orders for heavy-duty trucks returned to trend in May, falling to 23,300 units (not seasonally adjusted), reports ACT Research.

"Despite falling nearly 40% from April, May was nevertheless the eighth consecutive month of strong Class 8 order activity," said Kenny Vieth, president and senior analyst at ACT. "It is worth nothing April's level of order intake was unsustainable, so the pullback in May's order volume was anticipated."

Even though orders were less robust and industry build rates continued to grow, the Class 8 backlog grew for a ninth straight month in May, added Vieth.

ACT is the a leading publisher of new and used commercial vehicle (CV) 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.