To be eligible for a voucher, the old vehicle has to be less than 25 years old, and must have had average combined fuel economy of 18 mpg or less when new. The car must be drivable, and it must have been insured continuously for a year prior to the application for a voucher.

Someone who wants to trade a clunker for a new car must buy one that gets at least a combined 22 mpg, according to the Environmental Protection Agency.

To get a $3,500 voucher, a new car must be at least 4 mpg more efficient than the old one. To get $4,500, a new car must be at least 10 mpg more efficient than the old one.

For example, if you have a 1991 Acura Legend, (combined EPA rating: 18 mpg), you could use it to get $4,500 off a Ford Fusion hybrid.

If you trade an old vehicle for a new light-duty truck—a minivan, crossover or sport utility—the new truck must get at least 18 miles per gallon. For $3,500 the new vehicle must be at least 2 miles per gallon more efficient than the scrapped vehicle. A five-miles-per-gallon or better improvement on a light truck gets you $4,500. So that 1991 Legend could be worth $4,500 if you decided to buy a new, two-wheel-drive Honda CR-V, rated at 23 miles per gallon combined.

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