-- Japanese automakers have gained significant ground on domestic automakers in the U.S. government's $3 billion "cash for clunkers" program.

General Motors Co., Ford Motor Co. and Chrysler Group LLC saw a dropoff in sales while those of foreign rivals rose in the latest tally of "cash for clunkers" sales, as Toyota Motor Corp. dethroned GM as the top seller.

Toyota has sold 18.9 percent of vehicles purchased through the clunkers program, surpassing GM's 17.6 percent.

Through the first two weeks of the program, GM accounted for 18.7 percent of all vehicles sold, ahead of Toyota, which had 17.9 percent as of Aug. 5.

As of Friday morning, dealers had submitted voucher requests for just over $1.5 billion, representing 358,851 vehicle purchases, or about half of the money Congress has allocated.

Detroit's automakers combined sold 42.1 percent of all clunker replacements, down from an initial 47 percent of sales and slightly below their 44 percent U.S. market share.

The largest three Japanese automakers -- Toyota, Honda Motor Co. and Nissan Motor Co. -- saw their collective market share jump 2.7 percentage points to 39.2 percent of "clunkers" sales.

Ford is in third place with a 15.4 percent share; Honda is No. 4 with 12.9 percent. Chrysler's share fell to 9.1 percent, down from 10.6 percent Aug. 5. Nissan was in fifth place with 7.4 percent, while Hyundai Motor Co. was sixth with 6.7 percent.

Foreign automakers now dominate the top 10 list of vehicles sold through the program, with just two American vehicles among the leaders: the Ford Focus FWD in third and the Ford Escape FWD in seventh.

Both the Chevrolet Cobalt and Dodge Caliber fell off the top 10 list. GM spokesman Greg Martin said the program has been "a big sales driver for us." He believes the sales rankings will change.

"The sales will be a lot like the weather in Michigan in the springtime," Martin said. "It will change at any given time."

The program, officially known as the Car Allowance Rebate System, offers consumers vouchers worth up to $4,500 to junk cars with a combined fuel economy rating of 18 miles per gallon or lower.

Toyota spokesman Mike Michels said the company's inventory of fuel efficient vehicles had sharply fallen amid the strong demand. "The jury is still out" on which company finishes first in sales, Michels said. "There are a lot of variables at work, not the least of which is product availability, which is affecting everybody."

Erich Merkle, president of the industry consulting firm autoeconomy.com in Grand Rapids, said the rise in Asian sales was also due to product mix. "The compact car segment is really dominated by Asian automakers, and that's really one of the segments being helped the most by this program," Merkle said.

Several automakers, including GM, Ford and Toyota, have boosted production of fuel-efficient models. Toyota is running overtime at its New United Motor Manufacturing Inc. plant in California, known as Nummi, and its engine plant in West Virginia.

The Toyota Corolla retained the top sales spot overall in data compiled as of early Friday, followed by the Honda Civic. The Toyota Camry was fourth, followed by the Toyota Prius. Rounding out the top 10 was the Hyundai Elantra at No. 6, the Honda Fit in eighth, the Nissan Versa in ninth and the Honda CR-V FWD in 10th.

About halfway through the $3 billion program, 83 percent of trade-ins are trucks and 59 percent of new vehicle purchases are cars.

The average fuel economy of the new vehicles is 25 mpg, while the trade-in mileage is 15.8 mpg -- for an overall increase of 9.2 mpg, or a 58 percent improvement

Cars purchased under the program are, on average, 19 percent above the average fuel economy of all new cars available. Hybrids account for 4.5 percent of the vehicles purchased, compared to 3 percent of all U.S vehicle sales in June.

According to Transportation Department data, more than half of the top 10 new vehicles by volume purchased under the program were manufactured in the United States: the Toyota Corolla, the Ford Focus FWD, the Toyota Camry, the Ford Escape FWD and the Honda CR-V 4WD.

In total, 208,378 passenger cars and about 150,000 light trucks have been purchased under the program.

All top 10 traded-in clunkers are U.S. pickup trucks, SUVs and vans.

The program continues to be wildly popular in Michigan.

In the state, Michigan dealers have sought $80.6 million in vehicle vouchers -- third overall behind California ($152 million) and Texas ($91 million).

On Thursday, the Transportation Department approved plans to allow customers to order vehicles from automakers, a move that should benefit domestic automakers. Many dealers have reported shortages of fuel efficient U.S. vehicles, in part because GM and Chrysler sharply reduced production during their stays in bankruptcy.

Chrysler said it had a low inventory of eligible vehicles at the end of July.

"The company's plants are running at full production, and we are building vehicles for customer and dealer orders," Chrysler said in a statement Friday.

At the rate of spending, the clunkers program could be out of money by month's end. Last week, President Barack Obama signed a law adding $2 billion to the program, which was in danger of running out of money.

Also Friday, some dealers continued to complain that they haven't been paid for hundreds of thousands of dollars in voucher requests. The Transportation Department said it was working quickly to process voucher claims.

A government contractor is adding temporary workers to speed payment of vouchers.