the reason the new cars get bad gas mileage is because the big three have to meet environmental requirements. if you retuned the newer cars then you could get much better gas mileage than before.

i would be eager to see what the emissions output is of this experiment. not that i'm a green weenie, but in reality you can't compare new cars to rebuilt cars that really have no requirements to meet. i wouldn't doubt that you could get an older car to get better gas mileage by tuning it really lean and sacrificing power and emission to get economy.