U.S. and Canadian natural gas Class 8 truck retail sales started 2015 with slow growth and ended the year in the same fashion, according to a recent report from ACT Research.

ACT attributed this to the continuing decline of diesel prices, making the return on investment for adopting of natural gas less lucrative.

“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. “However, this doesn’t mean the adoption of NG fuel has stopped or that there are no new developments supporting a future uptick in NG truck orders.

“Despite sequential momentum slowing, with November NG heavy duty retail sales down 28% month-over-month and year-over-year, year-to-date volumes stand just 1% below 2014’s levels. ACT’s staff continues to closely monitor industry developments. 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.”

Additionally, the report provided examples of how equipment research and development efforts are continuing to advance the market. 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 on the horizon over the next few years.