U.S. sales of cars and trucks from Detroit's traditional Big Three U.S. automakers likely dropped in September as big employee pricing discount programs lost their allure with consumers and gas prices soared in the aftermath of hurricanes on the Gulf Coast, analysts said.

While U.S. automakers General Motors Corp. (GM) and Ford Motor Co. (F)are expected to report slumping sales Monday, their Japanese rivals, including Toyota Motor Corp. (search) , will post gains, analysts said.

Japanese automakers have been relentlessly increasing their share of the U.S. market, stealing sales from Ford and GM, which have been struggling with issues ranging from excess inventory to a shift in consumer taste away from larger sport utility vehicles.

"Payback is the best term to describe the slump in September sales," Burnham Securities analyst David Healy said.

"September's sales weakness may have been made a bit worse in unit terms by the direct and indirect effects of the two Gulf hurricanes," he said in a note, referring to Hurricanes Katrina and Rita.

Exceptionally strong sales in any one month can weigh on near-term future demand for mass-market brands like Chevrolet, Dodge and Ford, commonly referred to as the "pull-ahead" or "payback" effect, analysts said.

GM, which since June 1 has been selling 2005 model-year vehicles at the same low prices its offers to employees, could see sales decline as much as 28 percent in September, Merrill Lynch analyst John Casesa said, adding the automaker is giving back nearly all of the market share gains of June and July.

In the first two months of the summer that GM offered employee pricing rebates, sales at the world's largest automaker rose 41 percent and 20 percent, respectively. However, sales retrenched nearly 17 percent in August.

Analysts are forecasting that industry sales on a seasonally adjusted annual basis will dip below 16 million vehicles in September for the first time in 14 months. They expect the sales rate to be in range of 15 million to 16 million units, well below the 17.4 million rate last year and 16.8 million sold in August.

"While a deceleration is widely anticipated by investors, we believe the magnitude may come as a surprise," Deutsche Bank analyst Rod Lache said in a research note.

Sales at Ford, the No. 2 U.S. automaker, are expected to fall as much as 30 percent, while analysts' forecasts for sales of DaimlerChrysler AG's (DCX) Chrysler division range from unchanged to a slight decrease.

Both Ford and Chrysler have also been offering employee pricing discounts since July.

"Ford's incentive program, launched one month after GM's, has also gone kaput, but its hot-selling new cars — Mustang, Fusion — are providing an offset to weakening SUV and pickup truck sales," Casesa said.

Sales of large gas-guzzling sport utility vehicles have declined significantly this month as U.S. gasoline prices soared well over $3.00 a gallon in many parts of the country, analysts said.

"Now that the impact of the employee price discount programs has faded, and gas prices have escalated dramatically in the wake of Hurricane Katrina, we are seeing a strengthening of the trends that existed back in the spring," said Tom Libby, senior director of industry analysis at the Power Information Network, part of industry tracking firm J.D. Power and Associates.

"Customers are moving to smaller vehicles — both smaller SUVs and smaller cars — and this is playing to the strengths of the offshore-based manufacturers and hurting the domestics, which have relied, to a greater extent, on the larger vehicles," he said.

Japanese automakers Toyota, Nissan Motor Co. Ltd. (search) and Honda Motor Co. Ltd. (search) , which have been grabbing market share from the two leading U.S. automakers, are collectively expected to see a 9.6 percent increase in sales from a year ago, Casesa said.

Toyota's September sales will post a double-digit percentage gain, U.S. chief Jim Press said at the Reuters Autos Summit in Detroit last week. Nissan sales also should show a double-digit increase, in the range of 10 to 15 percent, Jed Connelly, Nissan's top U.S. sales executive for North America, said at the summit.

Toyota gained 2 points of retail share, while Honda's U.S. share rose 2.7 points during the first two weeks of September, according to J.D. Power.

Toyota continues to see strong demand across a broad range of car segments, including the large Avalon sedan, the Prius fuel-efficient hybrid car and the youth oriented Scion-brand vehicles.