The declines of U.S. auto sales could be a sign that sales are reaching a plateau after six years of growth. (Keith Srakocic/AP)

U.S. auto buyers are tapping the brakes.

May is usually one of the strongest months of the year for the U.S. auto industry, as Americans buy vehicles ahead of summer road trips. But most major automakers reported lower sales for the month compared to the same month a year ago.

General Motors’s sales fell 18 percent, Ford’s were down 6 percent and Toyota’s sales dropped 10 percent. Volkswagen’s sales were down 17 percent. Honda’s sales dropped 5 percent; Nissan’s fell 1 percent.

Fiat Chrysler and Subaru bucked the trend, with sales up 1 percent each.

The declines could be a sign that U.S. sales are finally reaching a plateau after six straight years of growth — a streak not seen since the 1920s. Sales rose 6 percent between 2014 and 2015, but are only up around 1 percent so far this year. Kelley Blue Book expects them to remain flat this summer.

Flattening sales could be good for consumers, at least in the short term. Automakers are offering more discounts in order to keep growing their U.S. market share. TrueCar.com estimated that incentive spending rose 7 percent in May to an average of $3,034 per vehicle.

But those discounts can hurt resale values and automakers’ profits, so companies must be careful as they deal with a dip in demand.

Results from early in May were enough for LMC Automotive, a forecasting firm, to lower its full-year prediction. LMC said it now expects total U.S. sales of 17.7 million vehicles this year, down from its previous target of 17.8 million.

But even 17.7 million vehicles would break the record of 17.5 million that was set last year.

“The sky is not falling,” said Michelle Krebs, a senior analyst with Autotrader.com.

This May also had one less weekend than last May, which could account for some of the decline. Ford said sales were strong over Memorial Day weekend.

GM’s sales dropped to 240,450 vehicles. Its Chevrolet and Buick brands saw the biggest declines; sales of the Chevrolet Silverado pickup, its best seller, were down 13 percent. GM blamed tight supplies of new products, including the small Chevrolet Cruze, as well as planned reductions in sales to rental car fleets. GM’s rental car sales were down 49 percent.

Ford said sales of its F-Series pickup rose 9 percent, while its luxury Lincoln brand saw a 7 percent sales increase. But that couldn’t make up for a 26 percent decline in its car sales. Ford’s SUV sales were flat. Ford’s overall sales dropped to 235,997.

Toyota said its U.S. sales fell to 219,339. Its car sales dropped 16 percent while its SUV and truck sales fell 2.5 percent. Sales of the Prius hybrid plummeted 36 percent, a victim of low gas prices.

Honda said its sales fell to 147,108 vehicles. Honda brand sales fell just 3 percent, and the HR-V subcompact SUV saw a 15 percent sales gain. But sales at Honda’s luxury Acura division dropped 20 percent.

Volkswagen’s brand sales dropped to 28,779 as the fallout continued from the German automaker’s emissions scandal.

Fiat Chrysler’s sales rose to just more than 204,000 vehicles, giving it its best May in 11 years. The all-SUV Jeep brand led the way, with sales up 14 percent. But Chrysler brand sales fell 19 percent as car sales faltered, and the Dodge brand dropped 5 percent. Ram pickup sales fell 3 percent.