Do not understand how Alternator low voltage load increase MPG.

[GasSavers_HAHA]

I kind of like the idea of using a Sterling engine to drive the alternator.
The Sterling would re-use the waste heat from the engine and cause little loss of efficiency. The numbers seem to work too for a normal car.

For example: 15 hp is used for driving the car forwards which means about 45 hp is wasted as heat. If we could suck up 25% of this for the Sterling engine (11,25 hp) and we managed 20% efficiency (very realistic), we still get 2.25 hp to drive the alternator, which seems about right.

I saw a reference to a system like this somewhere but it was for big trucks.

With some optimizations, the total efficiency would be much better and we would get an electrical surplus. With a hybrid system (like Hondas' IMA) we could use the power to offload the combustion engine even more.
The biggest practical problem is the availability of high output, low weight, Sterling engines. There really aren't any smaller ones above 1000W on the market It will also be much easier to experiment with this kind of system in a boat than a car due to space constraints and legal matters...