Retail sales of U.S. and Canadian natural-gas powered Class 8 trucks improved modestly in February after getting off to a slow start in 2016, according to a report issued by ACT Research.

However, while those sales ticked up month over month, they remain down both year-over-year and year to date, according to the research firm. Despite a 3% month over month uptick in February, year-to-date volumes are 14% below 2015’s level and year-over-year sales are down 25%.

ACT’s Natural Gas Quarterly attributes the positive February result to a high number of natural gas vehicle repeat sales. This is despite the fact that the continuing low price of diesel is making the return on investment for adopting of natural gas less lucrative for fleets -- unless they are already invested in natural gas-fueled vehicles.

Ken Vieth, ACT senior partner and general manager, explained that “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.”

But he cautioned that the situation doesn’t mean the adoption of natural gas has stopped or that there are no new developments supporting a future uptick in such truck orders.

“NG infrastructure continues to be built, albeit at targeted locations, and existing NG equipment users remain committed to its long-term viability and emission benefits,” he continued.

In addition, the report provides examples of how equipment research and development efforts are continuing to advance the market. At the same time, ACT Research said it “sees only modest, single-digit growth for the adoption of natural gas as a transportation fuel in the U.S. the next few years, barring legislative changes.”

The Natural Gas Quarterly provides information on the current and projected status of those factors that impact a decision to adopt natural gas. It includes a “dashboard gauge” that looks at fuel price spread, public heavy duty NG fueling infrastructure, NG equipment, and actual NG heavy-duty truck sales.

ACT offers a free quick reference calculator for fleets considering moving from diesel to natural gas.

Related: Report: CNG Competitive Against $2 Diesel