I like the idea of swapping tires to experiement with different ratios. That might be a good cheap way to sort it out if you can find a variety of tires (mounted on wheels) that you can borrow.
Consider the transmission specs on a recent 6-speed Corvette. 5th and 6th are both overdrive gears with 5th being about .7 and 6th about .5. In that evnironment (I beleive) you rarely use 6th unless you're running highway speed and expect to stay there.
But when it is appropriate you can cruise with the motor turning real slow. While it is just a 700r4, my K2500 implements a similar concept, without requiring me to think about what the motor is doing. I'd be happy to try a 6-speed Corvette but haven't had a chance...
