But a closer look suggests that the industry may be headed for choppier waters than the hoopla in Detroit would indicate. While sales are healthy, consumers are actually buying fewer new vehicles. Purchases by individual customers at dealerships — known as retail sales and considered the most accurate reflection of demand — declined slightly in both 2016 and 2017. Some automakers are offsetting lower consumer purchasing by selling more cars to fleets like rental-car companies.

More worrisome is that the drops in retail sales have come even as manufacturers have resorted to heftier discounts, which eat into their profits. Sales incentives are now equal to more than 11 percent of the average vehicle’s sticker price. As recently as 2014, that figure was below 8 percent.

There are other troubling signs, too. Interest rates have started rising, which increases the cost of financing or leasing a new car. Younger buyers are showing less interest in owning cars than older generations. And the supply of low-mileage used cars is growing, giving shoppers attractive and lower-cost alternatives to new cars. Close to four million leased vehicles will be turned in and offered for sale as used models this year, up from 3.6 million in 2017.

“There’s a lot of headwinds out there,” said Mark Wakefield, global head of the automotive and industrial practice at Alix Partners, a consulting firm.

The auto industry has a long history of going from boom to bust — periods of rising sales and buoyant profits followed by inevitable sales slumps that leave idle plants and mounting losses. The last bust coincided with the 2008 financial crisis and nearly ruined Detroit. General Motors and Chrysler had to be saved by federally engineered bankruptcy proceedings.