WASHINGTON -- The U.S. Transportation Department said it expects to issue final rules for the cash-for-guzzlers program around July 23 and warned dealers that they are legally responsible for any trade-ins structured before that date.

The federal guidance seeks to clear up some confusion that has arisen over when the $1 billion program goes into effect.

The legislation, signed into law last week by President Barack Obama, says consumers who swap their vehicles for new, more fuel-efficient cars or trucks are potentially eligible for federal credits starting Wednesday.

But the Transportation Department only started working last week on regulations putting the legislation into effect. It has 30 days to fashion the new rules.

The July 23 date for issuing the rules, announced today, is the government's first specific estimate of when the program will begin.

"Interested dealers and consumers may want to wait until all of the detailed issues that must be addressed in the implementing regulations are resolved," the department said on its cars.gov Web site for the program.

"If a dealer chooses to structure a transaction before the final rule is issued, they will bear the risks associated with later demonstrating that the transaction meets all of the specifications of the final rule," the site says.

Violators are subject to fines of up to $15,000 per transaction, according to the site.

The program seeks to spur U.S. auto sales while improving the fuel economy of vehicles on the road.

Consumers will be eligible for $3,500 or $4,500 credits if their current cars or light trucks get 18 mpg or less and the new vehicles they are buying get certain amounts more.

Dealers will be expected to verify that both the vehicle being traded in and the one being bought are eligible under the program. That will involve making sure that the registration and insurance information is accurate and that the old vehicle is drivable, the federal Web site says.

Dealers also have to confirm the fuel economy of both vehicles, the Transportation Department says.

The National Automobile Dealers Association today endorsed the stern guidance issued by the government.

"We wholeheartedly endorse caution in proceeding with the program until final rules are set in stone and dealers understand how to participate and comply with the program," NADA spokesman Bailey Wood said in an interview.

The program will continue until Nov. 1 and is expected to fund 250,000 vehicle swaps.

Dealers will be allowed to use the federal credits in addition to any advertised rebates or discounts, the Web site says. The credits also can be combined with other state and federal incentives, such as hybrid-vehicle credits.