If only I'd had the forethought to buy a less fuel-efficient car

My 1994 clunker got 19 mpg when it was new. According to cars.gov. That's 1 mpg too many. If you want to conserve gas, you should convert to gallons per mile (or gallons per 100 miles) and decrease that more-relevant number. Assuming they drive the same amount, trading in a car that gets 10 miles per gallon for one that gets 12.5 miles per gallon saves the same amount of gas as going from 20 mpg to 33.3 mpg or from 40 mpg to 200 mpg:

1/10mpg = .1 gallons per mile
1/12.5mpg = .08 gallons per mile
.1gpm - .08gpm = .02 gallon savings per mile

1/20mpg = .05 gallons per mile
1/33.3mpg = .03 gallons per mile
.05gpm - .03gpm = .02 gallon savings per mile

1/40mpg = .025 gallons per mile
1/200mpg = .005 gallons per mile
.025gpm - .005gpm = .02 gallon savings per mile

I think that in order to qualify for the "cash for clunkers" your new car should pass the following 3 criteria:

1) the new car gets at least 4 more mpg than the old one.
2) the new car gets at least 20 miles per gallon
3) the new car uses at least one fewer gallons per 100 miles

This graph illustrates these three criteria and their max, whicx is my proposal: