Part of high mileage is getting the engine to turn slower. I am in Illinois where my brother, Bruce has made a 39 T rear sprocket and installed a 17 T front for his new Serow. Pirelli street tires.
Sitting upright, no fairing whatsoever, first, thru 5th gear are just fine. It will maintain 60-65 with the wind but will not hold that speed into wind or up any hill.
This is a good beginning. Anything we get now in mileage improvement will come from streamlining.
Craig Vetter, Designer