[Logic]

There used to be a gizmo that disabled the alternator (and aircon) while accelerating advertised in the back of the South African Car Mags.

Shouldn't be too difficult to make something similar. I'm thinking a lever/switch that hangs down from a hinge that swings back when accelerating and down/forward when cruising and decelerating.

Then one would have to gear down while decelerating to use the car's momentum to turn the engine and alternator..

That's a good idea anyway as fuel injection systems today cut the 'idling fuel' completely when one does so.