Despite a good start to the year by General Motors Corp., fewer than half of American consumers -- 48.9% -- bought new cars and trucks in the first quarter this year from Detroit automakers, according to retail sales data provided by the Power Information Network.

Retail sales are purchases made directly by consumers in showrooms, and they exclude fleet sales to rental car companies, businesses and governments, which are typically sold in bulk at a discount.

Industry experts view retail sales, which represent about three-fourths of the industry's 17 million sales, as one of the best measures of market demand and the future financial performance of automakers, because they are generally more profitable sales.

The new low point in Detroit's share of the retail market is the result of a long-term consumer move away from their brands that seems to have picked up speed since September, fueled by a housing market slowdown, high oil prices and shaken consumer confidence.

While GM had the strongest quarter among metro Detroit automakers, Ford Motor Co. and DaimlerChrysler AG lost important retail market share to foreign competitors -- led by Toyota Motor Corp., which managed to drop its incentives to below $1,000 per vehicle on average and still snap up a substantial number of new customers.

"I think, in the near term, both Chrysler and Ford will continue to lose share, and that aggregate loss will more than offset any possible gain by GM," said Tom Libby, senior director of industry analysis at PIN, a subsidiary of J.D. Power and Associates.

"I don't expect the domestics' share to move back up above 50% this year."

Although Detroit could make a comeback, the domestic retail sales performance has been consistently on the decline for some time.

In 2005, Detroit's automakers had 54.5% of the retail market. By the end of last year, that had edged down to 50.1%.

Now, Detroit is down to 48.9%. That's a 1.2-percentage-point decline from the fourth quarter of 2006, and it's an even larger 2.1-percentage-point decline from the first quarter of last year.

These declines, measured to the tenths of a percentage point, might not seem like much. But each point of retail share keeps about one half of an assembly plant running.

Libby said even a half of a percentage point is considered an admirable gain in today's marketplace and a full percentage point is like "a huge mountain."

Just where is the bottom of this retail sales slide? Have we reached a tipping point where American car makers will be irrelevant in the American market?