If you missed out on Washington's cash incentive program to trade in your old clunker, Uncle Sam still has a deal for you: The government will sell you rejects from its own fleet, even as it makes dealers scrap all those old cars that were collected from the public.

The sale of federal castoffs at auction is nothing new; deals for the consumer mean income for the government. But in swapping out old government cars for new models under the economic stimulus package, officials also are claiming environmental benefits that don't add up.

The General Services Administration used stimulus money to buy 17,246 new vehicles, including more than 3,000 hybrids, for an impressive 40 percent improvement in fuel efficiency over the old models, the agency says.

It said that translates into a decline of 334 million pounds of greenhouse gases over the next seven years. But the environmental claim doesn't take into account that most of the old wheels still will be on the roads, driven by people who bought them at auction.

Just how that affects the environment depends on the gas mileage of the auctioned cars and whether buyers stopped driving worse clunkers when they bought government hand-me-downs. Mileage driven by new owners and government drivers also is a factor.

GSA officials said in a statement that greening the federal fleet helps the environment and fulfills the agency's “mission to serve as a responsible steward of taxpayer dollars and assets.”

“At a value of approximately $40 million, the government-owned replaced vehicles will not go to waste,” the agency said. “They will be resold to further invest in greening the federal fleet.”