Retail sales of Class 8 natural gas trucks in the U.S. and Canada improved in May, after getting off to a slow start earlier in the year, according to a report published by ACT Research.

The majority of natural-gas vehicle sales were attributed to repeat sales as well as purchases made by transit bus and refuse truck operators, according to ACT.

The lower price of diesel fuel has stunted the return on investment fleets might have for gained by adopting the alternative fuel, making the turn toward natural gas a less lucrative prospect for fleets.

Sales were up 48% over the previous month. But they are down 21% over the past 12 months and 24% year-to-date compared to 2015.

“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 remain lengthy,” said Ken Vieth, ACT’s senior partner and general manager. “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.”

Infrastructure is another barrier to entry for natural-gas vehicle adoption and while infrastructure continues to be built, it tends to be at a slower pace and at targeted locations. However, natural-gas equipment users have stayed committed to the long-term viability and emissions benefits, according to Vieth.

ACT Research published the information in its Natural Gas Quarterly report, which provides information on the current and projected status of factors that affect the decision to adopt natural gas vehicles. ACT also offers a quick reference calculator for fleets considering moving from diesel to natural gas.