Edmunds forecasts that 1,588,185 new cars and trucks will be sold in the U.S. in March for an estimated seasonally adjusted annual rate (SAAR) of 17.2 million. This reflects a 25.3 percent increase in sales from February 2019, but a 4 percent decrease from March 2018. Sales in the first quarter of 2019 are expected to be 122,000 units lower than in 2018, a 3 percent decrease.

"We can now confidently say new vehicle sales are past their peak; the question now is what the new normal will be," said Jeremy Acevedo, Edmunds' manager of industry analysis. "With new vehicle prices continuing to rise, interest rates sustaining post-recession highs and leasing growing increasingly expensive, pressure on the market is mounting."

Edmunds analysts say that if auto sales continue to decline for the rest of 2019 at the same rate they did in the first quarter, the industry could see a significant reduction compared to 2018.

"It's still early in the year, so there could be some surprises," Acevedo noted. "If inventory gets out of hand, it's possible that automakers will react by pumping up incentives, which could give sales a bit of a boost. But with the possibility of an economic downturn and potential tariffs looming, it seems certain new vehicle sales are settling into a slower pace."

Edmunds estimates that retail SAAR will come in at 13.7 million vehicles in March 2019, with fleet transactions accounting for 20.4 percent of total sales. An estimated 3.7 million used vehicles will be sold in March 2019, for a SAAR of 39.2 million (compared to 3.5 million — or a SAAR of 39.3 million — in February).