Just a thought but it seems to me that the reason for the Cash for Clunkers program is to help reduce our use of oil and therefore our dependence on foreign oil.
To this end something like the following would seem to make sense. Using 18 mpg as a base - which they are using now - how about if you were to get a refund of $1,000 if you bought a new car that got 10 mpg more than the base, $2,000 for a car with 20 mpg over the base, $3,000 for 30 mpg over and $4,000 for 40 mpg over the base (Or 58 mpg, not sure if that is available yet or not.) This way we would get more value for money by encouraging people to get the more gas efficient cars on the road. Just a thought.