The mathematical equation for high mpg is high compression + high torque + low rpm = higher mpg. That's why diesels and hybrids do so well. Todays 4 cylinders are race engines to get high mpg, they are running high compression again which you can with poor fuel and a computer.

The problem with older cars is rpm, I'm running a RAI 400 with 3:42 rear and a stock 4 sp, I get 8-12, I do 1000 miles a year and I am generally cruising at 2000rpm, I don't drive it enough to care, but if you got a 5 sp or an electronic over drive kit and went easy you'd be @20