My head is hurting. If bypassing the alternator is the secret to better mileage then we would all be riding around with solar panels on our roof by now. The engine doesn't care if the voltage regulator is saying "juice" or "no juice" because it is spinning anyway. It doesn't add a load like an a/c compressor does when it kicks on. Anyone ever feel a dip in horsepower when you turn on your lights? If so its time to worry about a new alternator or battery, not your mileage. The only benefit that you'd see from alt/no alt is to remove the belt or alt entirely so there's less load on the engine. Or, if you're feeling froggy, try hooking up an a/c compressor-style clutch to it. Of course, every alternator I've ever held outside of a car spun pretty freely, so I can't imagine you getting much of a gain there. Plus, now you're talking about the extra mass of the clutch assembly itself.
Let's look more at the real enemy-friction losses and weight. If you want better mileage start by shedding anything unnecessary: seat cushions, trim pieces, swap all that glass for plexi and, while you're at it, mount it flush with the pillars. Don't worry about putting the windows down, that will cost you in aero flow. Replace your heavy seat frames with folding chairs (it worked for me in the Beetle camper, recall) and now let's talk about friction.
We estimate that, in a Vee, each bump up in oil thickness uses .5 horsepower. Thats 1.5 HP in an engine that makes only 60HP, so you know I'm using the thinnest the weather will allow! That also eats gas. Red Line makes some fantastic low-friction transmission and rear-end fluid that protects like dope but flows like 10-30. Put your tires at the max pressure printed on the sidewall and get some full-moon wheel covers. Got a roof rack? Lose it. Tape over any cracks or seams you're not using.
This is scratching the surface and not even talking about driving techniques...
