DURABLE GOOD PURCHASES ARE POSTPONABLE. You saw it here first. (With fifteen more minutes of fame.) Subsequent events give me no reason to reconsider.

DETROIT — When August auto sales come out Wednesday, it isn't going to be pretty.

The year-over-year sales decline for the month will look terrible — down an estimated 30% — because the comparison will be with the buying frenzy fueled last August by the government's cash-for-clunkers program.

That's just the facts. Jeff Jacoby (via Betsy's Page) piles on.

But an even bigger part of the answer is that the supply of used cars is artificially low, because your Uncle Sam decided last year to destroy hundreds of thousands of perfectly good automobiles as part of its hare-brained Car Allowance Rebate System — or, as most of us called it, Cash for Clunkers. That was the program under which the government paid consumers up to $4,500 when they traded in an old car and bought a new one with better gas mileage. The traded-in cars — which had to be in drivable condition to qualify for the rebate — were then demolished: Dealers were required to chemically wreck each car’s engine, and send the car to be crushed or shredded.

Congress and the Obama administration trumpeted Cash for Clunkers as a triumph — the president pronounced it “successful beyond anybody’s imagination.’’ Which it was, if you define success as getting people to take “free’’ money to make a purchase most of them are going to make anyway, while simultaneously wiping out productive assets that could provide value to many other consumers for years to come. By any rational standard, however, this program was sheer folly.

Thirty years ago, a temporary increase in gasoline prices induced vehicle replacements that cascaded older guzzlers and polluters into the secondhand market, where the biggest cars, there being relatively few passenger trucks those days, became bargain used cars for poorer people, less likely to maintain their cars. Cash for clunkers prevented that cascade.

To be sure, Cash for Clunkers gave a powerful jolt to car sales in July and August of 2009. But it did so mostly by delaying sales that would otherwise have occurred in April, May, and June, or by accelerating those that would have taken place in September, October, or later. “Influencing the timing of consumers’ durable purchases is easy,’’ Edmunds CEO Jeremy Anwyl wrote a few days ago in a blog post looking back at the program. “Creating new purchases is not.’’ Of the 700,000 cars purchased during the clunkers frenzy, the estimated net increase in sales was only 125,000. Each incremental sale thus ended up costing the taxpayers a profligate $24,000.

Even on environmental grounds, Cash for Clunkers was an exorbitant dud. Researchers at the University of California-Davis calculated that the reduction of carbon dioxide attributable to the program cost no less than $237 per ton. In contrast, carbon emissions credits cost about $20 per ton in international markets.

Tell us what you really think, Jeff.

When all is said and done, Cash for Clunkers was a deplorable exercise in budgetary wastefulness, asset destruction, environmental irrelevance, and economic idiocy. Other than that, it was a screaming success.

There is a doctoral dissertation in here somewhere, although its title will be something along the lines of The Welfare Effects of Subsidizing Durable Goods Replacement. Sensitivity analysis of different policies for returning some clunkers to market will be difficult.