With falling fuel prices, U.S. and Canadian natural gas Class 8 truck retail sales had a slower 2015 compared to the previous year, according to ACT Research.

Retail sales of Class 8 natural gas trucks fell by 1% through November compared with the same period in 2014. The decline in diesel prices throughout 2015 has made the return on investment for adopting natural gas vehicles less lucrative, according the The Natural Gas Quarterly published by ACT.

“With the fuel price differential continuing to narrow, the ROI to convert from diesel to natural gas is moving in the wrong direction: payback periods are lengthening,” said Ken Vieth, ACT’s senior partner and general manager.

Despite falling sales this year, there is little evidence that natural gas adoption will stop or have a future surge in NG truck orders, Vieth added. ACT Research sees growth of the adoption of natural gas as a fuel for transportation in the U.S. but doesn’t expect to see double-digit sales expansion over the next few years.

“We’ve learned that despite the current fuel price differential, NG infrastructure continues to be built, albeit at targeted locations, and that existing NG equipment users remain committed to its long-term viability and emission benefits,” Vieth explained.