Orders of medium-duty vehicles in classes 5-7 surpassed the 20,000-unit level in December for the third time in the past four months with 21,600 units, according to preliminary data from ACT Research.

The last time classes 5-7 orders reached that benchmark came in the middle of 2006. Orders rose 15% from November. A year ago, December order volume fell 5% from November of 2014.

"December is typically a strong order month," said Kenny Vieth, president and senior analyst with ACT Research. "As such, seasonal adjustment lowers December’s medium duty net order volume to 20,500 units."

Meanwhile, trucks in classes 5-8 reached 49,700 units for a 40% increase month over month. ACT Research will post final numbers later this month.