DETROIT — Automakers expect little impact from the federal government shutdown, and they predict a fourth-quarter rebound after a rare sales decline in September.

Auto sales dropped 4 percent from a year ago to just over 1.1 million, mainly due to a calendar quirk that pulled Labor Day weekend transactions into August’s numbers. The drop ended a 27-month streak of gains for the industry. General Motors, Honda and Volkswagen reported double-digit declines for last month. Toyota, Nissan and Hyundai posted smaller decreases. Only Ford and Chrysler reported gains among the bigger automakers.

GM’s 11 percent drop was its first since July of last year. It allowed Ford to get within 2,049 vehicles of unseating GM as the top U.S. automaker for the first time since May of 2011.

Most industry officials viewed September as an anomaly. They also downplayed the impact of the government shutdown, assuming it’s a short one.

Kurt McNeil, GM’s U.S. sales chief, said the fundamentals are still in place for GM and the industry to rebound in the coming months. Jim Lentz, Toyota’s North American CEO, said people have grown used to dysfunction in Washington.

One reason GM’s sales fell last month was a reversal in pickup trucks, which have been hot-sellers. Sales of GM’s full-sized pickups, the Chevrolet Silverado and GMC Sierra, fell 8 percent even though the company is selling redesigned trucks.

Some versions of the trucks were slow to reach showrooms. At the same time, Ford’s F-series pickup, the top-selling vehicle in the nation, posted nearly a 10 percent increase, and sales of Chrysler’s Ram truck rose 8 percent.

Ford and Chrysler offered more than $4,000 in discounts as they sold down 2013 models, according to the Edmunds.com auto site. But GM reduced its incentive spending almost 30 percent compared with last year to about $3,900.

Pickup trucks are traditionally the top-selling vehicles in the U.S., and they’re key to automakers’ profits. Companies make around $10,000 per truck.

At Honda, sales dropped 10 percent as two of its most popular models, the Accord midsize car and the CR-V small crossover SUV, posted declines.

Ford bucked the industry trend with a 6 percent sales increase. It was led by sales of the Fusion midsize sedan, which jumped 62 percent over last September. Chrysler saw a 1 percent increase aided by the Ram and the Jeep Grand Cherokee with sales up 19 percent.