DETROIT — Auto sales defied a downcast economy in September, climbing 9.9 percent to their highest level in five months as new models arrived at dealerships and inventory shortages eased.

All three of the Detroit automakers reported gains, led by a 27.2 percent year-over-year increase for Chrysler, which outsold Toyota for the fourth time this year, reported The New York Times.

Toyota and Honda again trailed the rest of the industry, even though September was the first month since the March earthquake and tsunami in Japan that all of their plants were running at full capacity. Toyota’s sales dropped 17.5 percent, and Honda’s were down 8 percent. In contrast, Nissan sales increased 25.3 percent.

The industry’s seasonally adjusted, annualized selling rate rose to 13.1 million, the first time since April that they had exceeded 13 million. General Motors and Ford Motor each said they still expected total sales for 2011 to top 13 million, which would require demand to jump further in the fourth quarter.

Auto executives and analysts said shoppers had not been dissuaded by a declining stock market, bleak consumer confidence surveys, a sluggish housing market or high unemployment. Bigger discounts offered by some brands have helped, as have new offerings like the Chevrolet Sonic, a subcompact car, and a bevy of redesigned models from Chrysler.

“I don’t know of any other month where we had positive gains in auto sales with all of those negative factors,” Jesse Toprak, vice president for industry trends and insight at TrueCar.com, which tracks sales and pricing. “The automakers might be convincing some consumers who may not be so eager to spend their money to buy a car because the product is so compelling.”

Mr. Toprak said more consumers also were showing up at dealerships because their current vehicle had outlived its useful life and they had no choice but to buy a replacement. High used-car prices are prompting some in that situation to buy a new one instead.

“As long as things remain relatively stable, even in the face of persistently high unemployment, we’re going to consistently see slow growth,” Don Johnson, G.M.’s vice president for United States sales operations, said in a conference call. “Right now, the pent-up demand due to age of vehicles is what’s keeping this nice, steady, slow growth going.” G.M. sales increased 19.7 percent in September over a year ago.

Falling gas prices helped persuade more shoppers to buy pickup trucks and other larger vehicles. Sales of light trucks, including sport utility vehicles and minivans, rose 16.1 percent, while passenger cars were up 3.4 percent.

Sales of full-size pickups, which typically fare best when the construction industry prospers, surged 46 percent at Chrysler and 33 percent at G.M. Ford, whose sales were up 9 percent over all, sold 18 percent more light trucks but 8.7 percent fewer cars.

At Chrysler, September was the 18th consecutive month of year-over-year sales growth. It reported a 24.3 percent gain for its Jeep brand of S.U.V.’s.

“Irrespective of the economy, strong products equal strong sales,” Reid Bigland, Chrysler’s head of United States sales, said in a statement. “There is no double-dip downturn going on around here.”

Another carmaker with considerable momentum is Nissan, which experienced relatively minor disruptions from the Japan disaster. Nissan sold 68 percent more of its subcompact car, the Versa, and its midsize sedan, the Altima, has outsold the Honda Accord so far this year.

In addition, the Nissan Leaf, an electric car, added to its sales lead over the Chevrolet Volt, G.M.’s plug-in hybrid. Nissan sold 1,031 Leafs to G.M.’s 723 Volts, but G.M. officials said they were still ramping up production while expanding sales to dealers nationwide.

For Toyota and Honda, even though plants are running at full speed, inventories are expected to remain below prequake levels until early next year. But dealerships said they were finally able to meet most shoppers’ needs again, rather than just taking an order or hoping the customer came back later.

“It’s beginning to feel like normal, almost,” said Adam Skolnick, the general manager at Toyota of Watertown, near Boston. “We have plenty of cars on the lot, and I’m anticipating many, many more coming in the next 45 days or so.”

Mr. Skolnick said the arrival of the redesigned Toyota Camry sedan a week ago was helping the dealership make a quick recovery. For September, Camry sales fell 19.2 percent, but it was the industry’s top-selling car.

“It’s been like a bakery here, with people taking numbers to see the car,” Mr. Skolnick said. “It makes it feel fun again.”