In Defense of "Cash For Clunkers"

I keep reading critical articles about Cash For Clunkers(CFC) that focus on the weak mpg requirements.  I think these critics should do a little math.

Worst case scenario: If the owner of a clunker that gets 18mpg drives 15,000 miles per year and trades it in for a truck or SUV that gets 22 mpg, they will save 318 gallons of gas per year. In order for the owner of a 24 mpg clunker to save the same amount of gas, over the same number of miles driven, they would have to buy a new car that gets 48 mpg.
CFC isn’t perfect, but it’s not nearly as bad as mpg numbers make it seem. We need to start thinking of more useful rating systems for vehical efficiency.  Gallons per 10,000 miles driven makes sense to me.

CFC should also reduce auto accident injuries(newer cars are safer), and reduce smog forming emmisions, although the exclusion for cars older than 1984 limits the latter possibility.