If you've got an older car you'd like to trade in for a new, more fuel-efficient one, now's the time. A federal program that officially kicked off July 24 will hand you a tax-free credit of up to $4,500 for your junker if you buy or lease a more fuel-efficient new car.

The credit can be used toward the downpayment or to buy down lease terms on the new vehicle. The catch: With just $1 billion in funding, new-car dealers expect the money will go fast, possibly being exhausted in as little as a month.

Here are the basics on the Car Allowance Rebate System (CARS), more commonly known as the "cash for clunkers" program. Individuals and businesses can participate in CARS, says Department of Transportation spokeswoman Ellen Martin. There is a limit of one trade-in per any registered user--so a business owner could conceivably trade in one personal car and one registered to the business.

The new car can't have a purchase price of more than $45,000. Program rules require the dealership have the clunker scrapped.

The trade-in car must be 25 years old or less, be continuously registered and insured for the past year to the owner making the trade, and have a clear, unencumbered title. In some cases, the car can't be more recent than 2001 models.

For one category, classified as "work trucks," gas mileage isn't a factor and only a $3,500 credit can be obtained. There's a chart that helps you determine the amount of your CARS credit at http://www.cars.gov/files/day-one.pdf.

CARS officially ends Nov. 1, but there's reason to believe the money will run out long before then. It's simple math--divide $1 billion by $4,500 and you get less than 223,000 trade-ins before the money's gone. Since some trade-ins may only qualify for a $3,500 rebate, the money may go a bit farther. But any way you slice it, there will only be about 5,000 trade-ins per state, on average.

The money is already being tapped, as some dealers began making CARS deals weeks ago. Hyundai dealers started accepting CARS trade-ins July 1, as Hyundai Motor Co. decided to front dealers the federal credit until federal reimbursements become available. Other dealers began lining up clunker deals early, putting a "sold" sign on the new car in the deal and waiting to sign final papers.

In the New York City suburb of Mahwah, N.J., Rick DeSilva has done both types of early CARS deals. DeSilva, 57, owns two dealerships, Liberty Hyundai and Liberty Subaru, with combined sales of around $52 million a year.

His Subaru lot has parked several "sold" new cars, while his Hyundai dealership made 15 CARS deals before July 23, with early reimbursement from the automaker. DeSilva says sales at the Hyundai lot are up 20 percent in July, and he expects a bigger increase next month as CARS catches on.

"It's created a lot of interest and stimulated people to walk in," he says. "That gives us an opportunity to sell them."

Full details on the program, including a list of participating dealers, will be posted at www.cars.gov, or call the CARS hotline at (866) CAR-7891.