Gear ratio change on shaft drive?

Freely rolling chassis also contributes toward higher mpg.

Proper air pressure in tires.
Brakes not dragging.
Wheel bearings lubed and in good condition.
Chain in good condition (no kinks), properly lubed and correctly adjusted.

Good Fortune!

I have a fully built GPz 750 (810cc, pipe, pods, jet kit) and I have never gotten less than 35 mpg no matter how hard I ride.
Tire pressure is easy to check (bear in mind that the 1983 specs are no longer valid with modern day tire technology). Use the 10% rule. Hot tire pressure should be about 10% higher than cold for the front, around 15% higher for the rear. I run around 32 front, 36 rear. YMMV.
To check the front wheel dragging, put it on the center stand, put some weight on the rear seat to keep the front wheel off the ground (or have someone do it for you) and spin the front wheel. Listen for noises, and feel for drag.
You'll have a tougher time with the chain, though.