U.S. and Canadian natural gas Class 8 truck sales declined over a three-month period from June to August, according to the latest quarterly report from ACT Research.

Compared to the same three-month period in 2015, sales are down 29%. They are also down 25% year-to-date. On the buying side, natural gas vehicle purchases are mostly dominated by transit bus and refuse truck operators.

Lower amounts of freight and excess capacity have caused all vehicle sales, regardless of the type of fuel used, to be subdued. That has added to the built-in difficulties of the natural gas vehicle market, according to ACT.

“Regardless of the fuel used, heavy trucks are all competing for the same freight, which has failed to increase for at least the last five quarters,” said Steve Tam, ACT vice president.

“Coincidentally, many truckers increased their fleet sizes just as freight growth was slowing, setting up the current situation of excess capacity that is keeping the brakes on new truck sales, irrespective of fuel type," he added.