Orders of Class 5-7 vehicles rose 8% in September from a year ago and 18% from August at a time that typically sees lower numbers due to seasonal trends, according to preliminary estimates from ACT Research.

In September, orders reached 22,000 units, which brought the seasonally adjusted total for 2015 to 246,000 SAAR. Seasonal adjustment reduced September's net order volume to 20,500 units, according to Kenny Vieth, ACT's president and senior analyst.

"Class 5 enjoyed a 22% year-over-year increase, while Classes 6-7 grew 9%," Vieth said. "Strength in the Class 5 market was concentrated in trucks, while step van activity drove the incremental gain in the Classes 6-7 market."

The news came as falling Class 8 orders created a year-over-year decline in Classes 5-8. Vendors booked 40,700 orders in Classes 5-8 for September, which was 8% lower than a year ago.

"While we continue to believe that much of the order decline is a reflection of larger backlogs and tough comps, rather than any substantive change in demand, this has become a less compelling piece of evidence, as the spread in the year-over-year backlog has continued to narrow," Vieth said.