A big motor in a small, aerodynamic car with good gearing will get better mileage than a small motor in a small car running at high rpm.
I am in the process of swapping a 3800 into a sunfire. I have to get a different differential and drive chain/sprockets to get the gearing low enough, but I am hoping to get 40 mpg. It is an easy car to mod and weighs 2600 lbs.
If you can get your paws on a used corvette and get a better final drive, you would do well, since corvettes normally get 30-40 mpg when driven lightly.
