A "Cash for Clunkers" sign hangs in front of a Subaru dealership in Oregon City, Ore. Car and truck buyers looking to ditch their gas guzzlers are heading to dealerships to take advantage of the government's "cash for clunkers" program and buy more fuel-efficient vehicles.

NEW YORK — Car and truck owners looking to junk their gas guzzlers are flocking to dealerships to take advantage of the government's "cash for clunkers" program and buy more fuel-efficient vehicles, boosting sales in showrooms across the country.

"It's water to thirsty people," said Dave Kelleher, owner of two Chrysler dealerships outside Philadelphia. He said the number of shoppers has more than doubled at one of his dealerships and is climbing at the other.

"The last two days have been ripping," he said. The program — officially called the Cash Allowance Rebate System, or CARS — took effect over the weekend at the nearly 20,000 car dealers who have signed up with the Department of Transportation.

The program offers rebates of $3,500 to $4,500 for car shoppers who scrap their old vehicles to buy ones with better gas mileage.

Jim Aten, 68, was at a Toyota dealership in Gladstone, Ore., where he used the government rebate to get a peppy new Scion xB. The car gets about 28 miles per gallon on the highway compared with 17 for his old Ford Starcraft, a full-size van.

Without the incentive "I would probably keep driving it," Aten said. He said he was concerned that the government was spending money on the program, but acknowledged, "Maybe it will jump-start the car industry a little bit."

In Chattanooga, Tenn., the rebates generated about a dozen deals over the weekend for Mountain View Ford. "Most of them are truly clunkers," Mountain View President Clay Watson said of the big Expeditions and vans that customers have been trading in.

Here are the program's rules: The trade-in vehicle must be a 1984 model or newer and must get 18 mpg or less in the government's combined highway/city ratings. Buyers can get $3,500 for a new car if it gets at least 4 mpg more than the old one.

That rises to $4,500 if the new vehicle gets at least 10 mpg more. For SUVs, pickup trucks or minivans, owners can get a $3,500 rebate if the new vehicle gets at least 2 mpg more than the old vehicle, $4,500 if it gets at least 5 mpg more.