The government says the Cash for Clunkers program spurred nearly 700,000 sales of new cars this summer. That would seem like a boon for ailing automakers. But is it really?

The answer will hinge on what happens to auto sales in the coming months. And an analysis by car information site Edmunds.com suggests that it's going to be a lean fall for the likes of Toyota (TM), Ford (F), Honda (HMC) and Nissan (NSANY), makers of the most popular car models among consumers trading in old vehicles under the clunkers program.
Edmunds analysts see car shoppers' "current purchase intent," a measure of how likely they are to buy a car in the next 90 days, falling sharply now that the clunkers program has ended. It's now down 50 percent from its peak in mid-June. "People rushed into purchases that many would otherwise have made later this year," said Edmunds CEO Jeremy Anwyl in a statement. "The result will be lower sales in the weeks to come."

The problem seems simple: consumers who took advantage of incentives -- up to $4,500 toward a new, fuel-efficient car, after trading in an older gas guzzler -- would have bought cars sometime soon anyway. So instead of those sales being spread out over the coming months, Uncle Sam's limited-time offer concentrated them over the summer.

Now there are about 700,000 fewer potential car buyers, and a similar number of new-car owners. That shift will undoubtedly be reflected in the automakers' sales data in September and beyond.

There's one other place it'll show up, too -- in U.S. economic data. The Transportation Department estimates the clunkers program will goose third quarter gross domestic product by as much as 0.4 percentage points.

That gain will be illusory, too, except to the extent that Cash for Clunkers lured buyers who wouldn't have otherwise bought so soon -- probably a marginal segment of the Clunkers consumers.