Which cars qualify? To be eligible for the incentive, a buyer must bring in an operable car that is less than 25 years old and that gets a combined EPA city-highway average of 18 mpg or less. The buyer must have owned and insured the car for the last year — you cannot buy a clunker just to make the trade.

Economically speaking, the car also must be suitable for scrapping — if it is valued at more than $4,500, it may be worth more as an ordinary trade-in, since any car turned in under the program must be junked.

Kelley Blue Book has an online calculator at www.kbb.com/kbb/cash-for-clunkers/default.aspx.

 What do I do next? To get any incentive, you must buy a new vehicle that averages at least 22 mpg, and also at least 4 mpg more than the "clunker." To get the larger $4,500 incentive, the new car must beat the old one by 10 mpg.

One other requirement: The new vehicle's manufacturer's suggested retail price cannot exceed $45,000.

Some new cars obviously qualify — think Toyota Prius, Mini Cooper or Pontiac Vibe. So do most smaller cars.

But, depending on your current vehicle, so may some midsize or larger sedans, and plenty of non-hybrids, such as Toyota Camrys, Hyundai Sonatas, and four-cylinder versions of the Ford Fusion and Mercury Milan. To check models' mileage, go to www.fueleconomy.gov.

 Can I junk an SUV or minivan? Yes, and you may be able to use the incentive for a new one, too, which is one of the law's provisions that angers some critics. New SUVs and other vehicles classified as "light-duty trucks" under the law are eligible if they get at least 18 mpg.

For the $3,500 incentive, the new light truck must get at least 2 mpg more than the old one. For $4,500, it must get at least 5 mpg more.

Certain larger trucks weighing up to 5 tons also may qualify. For more information on eligibility and answers to other questions, go to www.cars.gov.

 Will this truly help the environment? Yes, but only at the margin.

House backers estimate average savings per vehicle of 250 gallons a year — plausible if you consider that a car averaging 15 mpg needs 1,000 gallons of gas to travel 15,000 miles a year, compared with 600 gallons for a car that gets 25 mpg.

At that rate, replacing 250,000 gas-guzzlers should yield an annual savings of 62 million gallons, assuming drivers will not drive more in more efficient vehicles, a premise some critics contest.

But if it is a step in the right direction, it is a small one. According to the Energy Information Administration, U.S. motorists consume 390 million gallons of gasoline each day.

Distributed by McClatchy-Tribune Information Services