"Cash for Clunkers" is an amazingly successful stimulus program, right? How else to explain its immense popularity? Not so fast.

Congress budgeted $1 billion to to provide car-buyers of rebates up to $4,500 when trading in a qualifying older vehicle for a new, more fuel-efficient one. The money was supposed to last into November, but it's already gone. Does this mean it worked? Not necessarily. As Jeremy Anwyl of explains, it appears the program shoe-horned months worth of car sales into a week or two, and may not have increased overall car sales much at all.

I love a good sales surge as much as anyone. But it's not that simple. First, it's not clear that cash for clunkers actually increased sales. noted recently that over 100,000 buyers put their purchases on hold waiting for the program to launch. Once consumers could start cashing in on July 24, showrooms were flooded and government servers were overwhelmed as the backlog of buyers finalized their purchases.

Secondly, on July 27, published an analysis showing that in any given month 60,000 to 70,000 "clunker-like" deals happen with no government program in place. The 200,000-plus deals the government was originally prepared to fund through the program's Nov. 1 end date were about the "natural" clunker trade-in rate.

Clearly, cash for clunkers was underfunded from the start. Consumers quickly figured that out and rushed to take advantage before funding ran out.

This sales frenzy was inevitable. We have crammed three to four months of normal activity into just a few days.

While automakers may like the program, there's little reason to believe it will contribute to an economic recovery -- and even less reason to think the program needs another $2 billion, as approved by the House of Representatives. More from the WSJ and Derek Thompson.