For cars, the milage is 22, not 18, according to this one website.
http://www.cashforclunkersfacts.com/
Summary of Cash for Clunkers Voucher Qualifications
Min. Fuel Economy for New Vehicle - Passenger Car 22 mpg
Mileage improvement of at least 4 mpg - $3,500 Voucher
Mileage improvement of at least 10 mpg - $4,500 Voucher
Light-Duty Truck ** 18 mpg *
Mileage improvement of at least 2 mpg - $3,500 Voucher
Mileage improvement of at least 5 mpg - $4,500 Voucher
Large Light-Duty Trucks *** 15 mpg *
Mileage improvement of at least 1 mpg or trade-in of a work truck - $3,500 Voucher
Mileage improvement of at least 2 mpg - $4,500 Voucher
Mileage improvement of only 1 or 2 miles per gallon for trucks? Are you freaking kidding me? grrrrrr that pisses me off! Why the fuck do TRUCKS always get off easy! They pollute the most and get the WORST mileage!