NEW YORK (CNNMoney.com) — The popular Cash for Clunkers program gave a strong boost to auto sales in August, putting the industry on course for its best month this year. But sales dropped sharply in the last week of August — after Cash for Clunkers ended.

Ford (F, Fortune 500) reported Tuesday that sales rose 17% compared to August 2008, its biggest jump in sales in four years. Still, Ford sales’ gain was short of the 22% increase forecast by sales tracker Edmunds.com. Ford shares were down nearly 4% following the release of the report.

Chrysler Group reported a 15% decline in sales from a year-ago, but that was still better than the 22% drop forecast by Edmunds.com. Chrysler’s sales were also 5% higher than they were in July.

But sharp declines in sales during the last week of August have raised doubts about the outlook for sales for the remainder of the year, said Edmunds.com senior analyst Jessica Caldwell.

Cash for Clunkers left dealers with limited inventory of new vehicles once the program ended and also with fewer buyers interested in buying cars now that they are no longer eligible for $4500 in rebates.

Caldwell said that the pace of sales went from a seasonally-adjusted annual rate of 15 million vehicles while the program was in effect in August to only about 8 million currently.

“Cash for Clunkers sent the sales rate on a wild roller coaster ride,” she said.

Although Ford did not mention the Cash for Clunkers program specifically in its release, Ford director of sales analysis George Pipas told CNNMoney.com Monday that sales were helped by the program, which rewarded purchases of more fuel efficient vehicles such as Ford’s popular Focus model.

Ford said in its release that sales were also lifted by signs of recovery in the U.S. economy overall. Sales of trucks and vans, for example, rose 12%. In a statement, Ford vice president of U.S. sales Ken Czubay said the company was hopeful that “small business owners are seeing signs of recovery and gaining confidence in the outlook for stronger business conditions.”

Chrysler said its sales were limited by low inventory of some of its vehicles. It has joined Ford and General Motors in announcing additional production of vehicles this fall in order to replenish dealers’ supplies.

Other automakers are expected to report a similar boost from Clunkers when they release August sales results later in the day. Pipas said Monday that the program may have lifted industrywide sales to their first year-over-year gain since October 2007. Edmunds.com is forecasting a 6% drop in industrywide sales compared to a year ago, however.

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