The National Highway Traffic Safety Administration is working to implement a new law that will provide up to $4,500 toward the purchase of new, fuel-efficient cars for people who trade in their old gas-guzzlers.

The Car Allowance Rebate System, or CARS Act, was recently signed by President Obama. It aims to preserve the environment by reducing greenhouse gas emissions, support America’s car industry and help consumers save money at the gas pump.

A list of participating dealerships will be posted on the Web site www.cars.gov by July 24.

“Only new car dealerships will participate,” said Tom Sherman of Usem, Inc. “It requires registration with the National Traffic Administration.”

The program will require trade-in vehicles to get no more than 18 miles per gallon (according to the “new” combined city/highway fuel requirements), be manufactured no less than 25 years before date of trade-in, be in driveable condition and be continuously insured and registered to the same owner for a full year before the trade-in.

If all requirements are met, the trade-in vehicle will be scrapped and the purchaser will receive a rebate on a new car.

For new cars getting four to nine more miles per gallon than the trade-in car, the purchaser will get $3,500; for cars getting 10 or more miles per gallon, the purchaser will get $4,500.

New vehicles may also be leased, but the lease period must be at least five years.

The official procedures for trading in a car haven’t been defined yet, but they will be published on www.cars.gov by July 24. NHTSA will also set up a Web site to help customers determine whether their vehicle is eligible for trade-in.

The program will stay in effect through Nov. 1, 2009, or earlier, if funds are exhausted.