Do not understand how Alternator low voltage load increase MPG.

[Snax]

You need to think outside the box a bit csrmel. I've seen a RWD driveshaft driven alternator work just fine. In this case, it was with an independent rear suspension, so the driveshaft did not move vertically. Likewise however, any transaxle that employs a carrier bearing and stub axle setup could easily do the same thing. In the case of the MR2, it would have to be driven off of the transaxle side of an inboard CV joint, but nonetheless, possible.

Alternator output must also be regulated to avoid overpowering whatever loads are connected - like a fully charged battery for instance.