While the glamour in auto sales is clearly in moving new metal, profits are usually higher on what the industry likes to call "previously owned," according to the New York Times.

A General Motors dealer might make as little as $300 profit on a new vehicle, according to Mark Schienberg, the president of the Greater New York Automobile Association in Whitestone, Queens, N.Y. The manufacturer controls the wholesale price, and the sticker price is pretty much frozen. The dealership tends to make its money on used cars, as well as from the F&I, service and parts departments.

The zero percent financing promotions have indirectly driven up the sales of used cars and light trucks, projected at 12.6 million this year, up from 12.5 million the year before, according to Paul Taylor, chief economist of the National Automobile Dealers Association (NADA).

In October, when the sale of new vehicles spiked at 1.7 million, more than half of the buyers brought traded in an old vehicle, flooding dealership inventories with an additional 212,000 used cars and trucks.

Other factors, including fleet-culling by rental companies and the return of leased vehicles, have helped to create what some people in the industry openly call a used-car glut.

Despite the large inventories, used-car prices at dealerships have not fallen but have increased nearly 2 percent, contrary to some reports, according to Taylor.

NADA expects the trend to continue, a reflection of the sluggish economy. "The myth is that people tend to buy used cars in a recession rather than new ones," Taylor said. "The reality is that car sales drop, but used cars drop slower than new ones."