DETROIT — U.S. auto sales fell slightly in September as demand for new cars and trucks sputtered.

Sales dropped 0.5 percent to 1.4 million in September, according to Autodata Corp. It was the fourth month of year-over-year sales declines so far this year.

Automakers reported mixed results Monday. Nissan’s sales rose 4.9 percent over last September. Hyundai and Subaru reported 4-percent sales gains, while Toyota’s sales rose 1.5 percent. General Motors’ and Honda’s sales were flat for the month. Fiat Chrysler’s sales fell 1 percent. Ford and Volkswagen both reported 8-percent declines.

Slowing sales aren’t necessarily bad news for automakers or consumers. Sales remain near historic highs, and some analysts suggest 2016 sales could still top the record of 17.5 million set last year. Favorable conditions such as low interest rates and low gas prices remain in place. Consumer confidence hit a nine-year high in September, according to the Conference Board’s index.

But after six straight years of growth, demand is clearly slowing. U.S. sales are up just 0.5 percent through September.

Cars have been particularly hard hit as low gas prices have made SUVs and trucks even more appealing. Brands without competitive new SUVs are struggling; Fiat, for example, saw its sales plummet 30 percent last month.

New cars also are feeling pressure from used cars because of the growing popularity of leasing. More than 30 percent of new vehicles are now leased, up from 20 percent five years ago. Around 500,000 off-lease vehicles are returning to the used car market this year and next year, and many consumers will choose them instead of new vehicles, says Alec Gutierrez, a senior market analyst with Kelley Blue Book.

That has automakers pushing harder for sales. Incentive spending hit a record of $3,923 per vehicle last month, surpassing the previous high set in December 2008 during the recession.

BMW was offering $6,732 in incentives per vehicle in September, up 44 percent from a year ago, according to Fiat Chrysler spent $4,302 per vehicle on deals, up 23 percent. Buyers could get $4,500 cash back on a 2016 Ford Escape or $2,000 on a 2017 Chevrolet Malibu.

Mark LaNeve, Ford’s U.S. sales chief, said incentive spending was $430 per vehicle higher than last September. Usually, he said, incentives are only about $100 higher or lower than the year before.

“The business is as competitive as I’ve seen in my 32 years,” LaNeve said during a conference call with analysts and media. “Everybody’s trying to protect their share and, in spots, grow it.”

Among the sales tallies from automakers:

— General Motors Co.’s sales fell less than 1 percent to 249,795. Buick sales jumped 14 percent thanks to strong sales of the new Envision small SUV and Cadillac sales rose 3 percent as the new XT5 SUV and CT6 sedan hit the market. But Chevrolet sales were flat and GMC sales fell 9 percent.

— Ford Motor Co.’s sales fell 8 percent to 204,447. The company said a planned 36-percent cut in sales to low-profit rental car companies was partly to blame. Ford’s car sales plummeted 21 percent compared to last September, led by a 39.5 percent decline for the subcompact Fiesta. F-Series truck sales also fell 3 percent as Ford prepared to launch its new Super Duty trucks. Lincoln sales were up 1 percent.

— Toyota Motor Co.’s sales rose 1.5 percent to 197,260. The company’s car sales were down 9 percent. Sales of the hybrid Prius fell 23 percent, a victim of low gas prices. But SUV and truck sales were up 13 percent.

— Fiat Chrysler Automobiles said its sales fell 1 percent to 192,883. Ram sales jumped 29 percent on strong sales of the Ram pickup and ProMaster van, but sales declined for the Chrysler, Jeep, Dodge, Fiat and Alfa brands.

— Honda Motor Co.’s sales fell less than 1 percent to 133,655. Sales of Honda’s new small SUV, the HR-V, were up 49 percent over last year, when it was just hitting the market, but the company’s car sales fell 8 percent. Luxury Acura sales were down 13 percent.

— Nissan Motor Co.’s sales rose 5 percent to 127,797. SUV and truck sales jumped 19 percent; sales of the Rogue small SUV rose 6 percent to more than 26,000, a September record. Luxury Infiniti sales were up 12 percent.

— Hyundai Motor Co.’s sales rose 4 percent to 66,610. Hyundai’s biggest seller, the Sonata midsize sedan, saw sales slip 6 percent, but sales of the Santa Fe SUV were up 5.5 percent.

— Subaru brand sales rose 4 percent to 54,918, led by a 12 percent gain for the Subaru Outback small SUV.

— Volkswagen brand sales dropped 8 percent to 24,112. Volkswagen said Tiguan small SUV sales rose slightly but sales of the rest of its vehicles were down as the company continues to feel the effects of a year-old diesel engine scandal.