Why is it that you are able to buy a gas-guzzling(18mpg) truck with the "cash for...

...clunkers" program? To get the program credit your trade in car has to get 18mpg or less but you are also able to purchase a car that gets only 18mpg. This doesn't make any sense to me! Trade a clunker for a clunker??? I bet a lot of folks are trading their trucks for new ones. I have a 98' Mustang that I consider a clunker but it's not eligible. If it were I would purchase an economy car that gets great mileage(over 30mpg). Can anyone offer some insight???