I have a 2002 Olds Silhouette with city of 18 and highway of 20 so average is 19...doesnt qualify

it should be sliding scale

i.e.

17 mpg 4500

18 mpg 4000

19 mpg 3500

20 mpg 3000

21 mpg 2500 and so on

and your new car must get say atleast 10 mpg better if more than 10 mpg add 50 dollars for every mpg over

example: I want a 2009 Prius mpg of 48 combined

so I get a 3500 federal tax credit plus another 950 (19x50) or a total of 4500.