NEW YORK — Lexus once again stands alone atop a closely watched ranking of vehicle dependability after Buick slipped from the No. 1 spot it shared with the Japanese luxury brand last year, J.D. Power and Associates says.

It’s the 14th straight year Toyota Motor Corp.’s high-end brand has held the top position in the annual study, which measures problems experienced by the original owners of vehicles after three years. Lexus had 120 problems per 100 vehicles, down from 145 last year.

“That’s a pretty good track record,” said Dave Sargent, J.D. Power’s vice president of automotive research. “They benefited to some degree … where a couple of their very important models in their second year on the market — the ES and the RX, which together account for over two-thirds of lexus sales — both improved significantly.”

Ford Motor Co.’s Mercury brand ranked second, followed by General Motors Corp.’s Cadillac. Toyota was fourth, and Honda Motor Co.’s Acura luxury brand was fifth. Land Rover, which Ford sold this year to India’s Tata Motors Ltd., was the worst-performing brand, with 344 problems.

The industry average improved to 206 problems per 100 vehicles, from 216 a year ago.

Buick, owned by GM, fell to sixth place in this year’s study with 163 problems, although its now-discontinued Buick Century was the top-ranked vehicle in the midsize car segment.

“The lower score is largely due to vehicles that are no longer in the marketplace,” Sargent said. “The vehicles are still out there, so the study is still relevant. But obviously they had some problems.”

GM’s Saab brand was the most improved in this year’s study, improving to 254 problems from 319. More than 60 percent of the 38 brands in the study improved from last year.

The No. 1 problem cited in the study, based on responses from more than 52,000 original owners of 2005 model-year vehicles, was wind noise, followed by noisy brakes, pulling to the left or right, dashboard issues and window fogging. The study weights all problems equally.

Vehicle dependability has been steadily improving across the industry overall, Sargent said. Since the 2005 study, the industry average has improved from 237 problems per 100 vehicles to 206 this year. That equates to slightly more than two problems per vehicle.

Furthermore, the types of problems reported have trended toward “soft” problems, like funny noises or aesthetic wear, in place of “hard” problems such as major technical defects, Sargent said.

Broken out by segment, Lexus took top honors in six categories for its IS 300, ES 300 and LS 300 sedans, the SC 430 coupe, and the GX 470 and LX 470 utility vehicles. Toyota led four categories and tied Honda for a fifth. Toyota’s Prius hybrid was the top-ranked vehicle in the compact car segment.

Besides the Buick Century, one other GM vehicle was ranked the best in its segment: The Chevrolet Monte Carlo took the honor amid midsize sporty cars. Ford led three categories with the Crown Victoria large car, the Ford Ranger midsize pickup and the Mercury Monterey minivan.

Chrysler LLC was the only Detroit automaker with no brands ranked better than the industry average.

TRAVERSE CITY, Mich. — General Motors said Thursday that it had “essentially finished” designing its first plug-in hybrid car, the Chevrolet Volt, and would have production-ready prototypes within 10 days.
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The automaker still has considerable work to do on the car’s lithium-ion battery and other technology in the two years before the Volt is scheduled to go on sale, but completing the design is a milestone for what is arguably the most crucial car in decades for G.M.

The Volt would be able to travel at least 40 miles on battery power alone, G.M. said. The battery is recharged by plugging a cord into a household outlet.

Bob Boniface, the director of design for the Volt, showed sketches of the car and photos of its front and rear corners at an industry conference in northern Michigan. He said G.M. had made the Volt more aerodynamic and attractive since displaying it as a concept car at the Detroit auto show in January 2007.

The changes, including a shorter hood and more rounded front end, have increased the car’s battery range by about 6 or 7 miles, Mr. Boniface said. By year’s end, G.M. expects to have 50 prototypes for testing.

The Detroit automakers have been criticized for making gas-thirsty vehicles, and the Volt has gained interest from consumers who see it as a way to save on gasoline.

As of Thursday afternoon, 35,750 people from all 50 states and 63 countries had signed up on an unofficial waiting list for the car at gm-volt.com, a Web site run by a neurologist in New Jersey who is not affiliated with G.M. The doctor, Lyle Dennis, started the site as a fan when G.M. announced the car.

G.M. has said it will charge $30,000 to $40,000 for the four-door Volt. Frank Weber, G.M.’s vehicle line executive for the Volt, said that the company did not expect to make money in the near term but that the “E-flex” battery technology will ultimately allow G.M. to sell a profitable line of ultrafuel-efficient vehicles.

G.M. aims to be the first automaker to sell a plug-in hybrid, but Toyota Motor also says it plans to introduce such a vehicle in 2010.