A better "cash for clunkers" program?

The Cash for Clunkers program will shortly be ending after a whirlwind history of stimulating many new car sales in just a few weeks (and also running out of money). But, as Michael Gerrard from Columbia University's Center for Climate Change Law points out, for the amount of money spent, the government saved relatively few tons of carbon. Could this program have been run more effectively, from an ecological perspective? One way to get more bang for the clunker buck would have been a higher standard on the minimum improvement in mpg needed to qualify for the program. Admittedly, it would have not led to the same economic shot in the arm, but there are probably enough buyers out there who would come forward for say, a 15 to 20 mpg improvement between the clunker going in and the fuel-efficient green car coming out, that eventually the government would have found outlets for the billions of dollars made available. Maybe not in just a few weeks, but over the course of 6 to 12 months, the fund would have been used up, and then the total amount of CO2 reduced from the program would have been much greater.