General MotorsGeneral Motors' share of the U.S. automobile market is expected to slip to 18% in September, the third lowest ever recorded by the Detroit automaker, according to statistics from online car-buying guide The site's forecast calls for GM to sell about 171,000 cars this month, 10.4% more than a year ago, but nearly 8% fewer than the 185,000 cars and trucks it sold in August.

GM recorded its lowest share of the U.S. auto market -- 17.6% -- in March 2010. Last month, the No. 1 U.S. carmaker held 18.6% of the market. At its peak of market power in 1962, GM made over 50% of all cars sold in the country.

GM's September market share slip comes as competitors Ford Motor (F), Toyota Motor (TM), and sister brands Hyundai and Kia are doing better. According to TrueCar, Ford is expected to gain 0.5% for a 16.3% share, while Toyota is expected to advance 0.6% to 15.6%. Hyundai/Kia is forecast to pick up 0.3% for a total 8.9% of the market -- its largest ever.

Meanwhile, Honda Motor (HMC) is seen maintaining its near-11% share, while Chrysler Group and Nissan Motors (NSANY) are expected to record slight decreases in market share, at 9.7% and 7.5%, respectively.

Overall, TrueCar expects automakers to sell about 951,000 vehicles this month, a 28% increase from a year ago, but 5% fewer than last month, on an unadjusted basis.

Automotive Value -- and Incentives

"Historically, the average drop is 10% from August to September, so the lack of a double-digit drop this month could be a sign of relative strengthening in the market," said Jesse Toprak, vice president of industry trends at TrueCar. "But consumers continue to seek value-oriented brands such as Hyundai or Kia due to a lack of clarity in the economy."

September's forecast translates into a seasonally adjusted annualized rate of 11.66 million new car sales, up from 11.47 million last month, TrueCar said. Analyst expectations call for automakers to sell 11.5 million to 12 million new cars and trucks in the 2010 calendar year.

Despite lower sales, automakers have eased off incentives, coinciding with the introduction of 2011 model year vehicles. Among the U.S.'s seven largest suppliers of cars and trucks, only Chrysler and Hyundai/Kia are expected to boost incentives this month compared to August, TrueCar said.

Toyota touched off an incentives war last spring to stem falling sales of its cars and trucks caused by massive safety recalls. The move was largely credited with reviving Toyota sales.

Overall industry spending on incentives is forecast to fall 3.2% from August to $2.55 billion, or an average $2,683 a vehicle. The decrease in incentives is attributed to better production management from manufacturers, as most late-model new vehicles move quickly off dealership lots, TrueCar said.

Among other findings, TrueCar expects sales of automobiles to consumers, or retail sales, to fall about 8% compared to last month. It also expects fleet sales, such as those to rental-car agencies, to make up about 21% of September sales.

Also, in its inaugural look at used-car sales, TrueCar said it expects transactions involving previously owned vehicles to drop sharply this month compared to August, down about 17% to nearly 3.3 million.

TrueCar said it has begun analyzing used-car sales, which include sales at franchise and independent dealers as well as private-party transactions, in conjunction with sales of new cars to provide better picture of the health of the U.S. auto industry.

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