LOS ANGELES – The service departments of auto dealers are headed for tough times.

That’s the conclusion of a report issued Wednesday by consumer research company J.D. Power & Associates, which also found that Toyota Motor Corp.’s Lexus brand ranks highest in customer satisfaction with dealer service among the luxury nameplates. Buick scored highest among nonluxury brands, after factoring out tiny producers Hummer and Saturn, which General Motors Co. is closing.

”It is mainly a function of lower auto sales. If sales were increasing or at least constant, dealers would have an influx of new owners to replace people they lose to independent service shops,” said Jon Osborn, research director at J.D. Power.

Automakers sold 10.4 million vehicles last year, down 21.2 percent from 2008 and the fewest since 1970. Most analysts expect auto sales to grow by about 10 percent this year to around 11.5 million, still far below the 16 million to 17 million vehicles sold annually from 1999 through 2007.

J.D. Power surveyed owners of vehicles purchased during a five-year period ending in 2009 and projected that their service department expenditure would drop by 25 percent between 2009 and 2013. Service visits are expected to fall by 19 percent between 2009 and 2013.

”Dealers are going to have to do something to get more customers in. More competitive pricing would be one way to do it. Going after the quick-lube business would be another way,” Osborn said.

Some will consider opening service facilities not connected to showrooms, a move to increase customer convenience, he said. Typically, gas stations, tire stores, quick-lube joints and other service options are closer to an individual’s home than the dealer where he or she purchased the vehicle, Osborn said. And the overhead for these independent shops is much less than that of the big dealerships because they have fewer amenities and less space.

In its 2010 customer service index, J.D. Power also looked at satisfaction among vehicle owners who visit a service department for maintenance or repair work. The rankings are based on dealer performance during the first three years of ownership, which J.D. Power said represents the majority of the vehicle warranty period.

Lexus topped the luxury makers for the second straight year with a score of 837 on a 1,000-point scale. Most of the research was conducted before Toyota issued 10 million recalls worldwide in recent months for a variety of acceleration and braking problems. Some Lexus models were caught up in the recalls.

Rounding out the top five in the premium segment are Cadillac (827), Jaguar (822), Acura (817) and BMW (816).

Among nonluxury brands, Hummer ranked the highest with a score of 815. It was followed by Saturn (808) and two other GM brands, Buick (805) and Chevrolet (787). BMW-owned Mini ranked fifth with a 786 score.

The study measured customer satisfaction in five areas, including service quality, service initiation, service adviser, service facility and vehicle pickup. It is based on responses from more than 114,200 owners and lessees of 2005 to 2009 model-year vehicles.