Orders of medium-duty vehicles increased 17 percent to about 19,000 in May to rebound from earlier in the year, according to early estimated from ACT Research.

The order signals strong demand in the sector, said Kenny Vieth, ACT's president and senior analyst.

"This was the sixth time in the past nine months that Classes 5-7 orders reached at least 19,000 units," said Vieth.

Meanwhile, Class 8 orders fell 8 percent to about 39,000 in May.