I would say the torque curve is most important thing to consider, once you drop below that "ideal" point, you mileage will suffer just as much or more. It takes a lot of power to push a brick at 70 mph. The gear has to be matched to the engine, for example, our 2001 VW Beetle 1.8L runs 3000 rpm at 70 mph because thats where the torque is.
My trucks fuel mileage didn't drop much at all when I went from 3.42 to a 3.08, the engine runs less rpm and sounds better at 70, but it takes more throttle to hold it there. Now at 55 I do see a gain.
Other then chopping the top off and dropping the frame to the ground, you will have to find a happy medium and in gears and rpm.
Joe
