Do not understand how Alternator low voltage load increase MPG.

[ajohnmeyer]

I'll give this a shot, but anyone feel free to jump in and correct me if I'm wrong.
well, it's not really voltage that you're trying to decrease, the alternators output voltage should remain relatively constant, it's the amperage that you're trying to keep down. The way I understand it is that the more amperage that you are demanding from the alternator (or any generator for that matter) the more physical resistance that the alternator is presenting to the engine. So, the more amps you draw, the more HP required to turn the alternator. So the overall HP load of the alternator should (relatively) match the amperage load of your electrical system.

It's not quite like the other main parasitic accessory the a/c compressor. With a compressor it's either on or it's off. When it's off there's only a slight load due to the increase in turning the extra belt and clutch wheel. When it's on there's a lot of resistance because you're having to compress all that freon, which takes a lot of HP to do.