I was just talking to a coworker this am about this.. He found with his stock 300ZX, if he cruises in 6th gear at 70, he gets roughly 2/3 of the milage he gets when he cruises in 5th...
Economy is all about the engine parts and driveline working together perfectly, and even in a modern computer controlled car, it's a floating target. IMO, for best economy, a clean, cotton air filter, well tuned carb and ignition, a good alingment, tires near their max pressure, and keep your foot smooth, is the way to get all you're going to get..lol.