[Big Dave]

As Post #9 showed this is an old idea. It works, within limitations. all you really have to do is deal with the mounting and sealing issues, and it will downspeed the engine (to use the term in vogue in automotive engineering circles) and probably improve MPGs.

Post #11 brings up a valid point. Automotive automatic transmissions rely on the torque converter to pump ATF to the cooler. If you downspeed the TC, you reduce oil flow to the cooler. You'd risk cooking the transmission and getting spectacular transmission failures.

I have the GV second-stage overdrive, and like in Post #12, I view it as sub-optimal but I am already down on the lowest ratios commercially available. I suppose I could substitute a Tremec 6060 with its 0.50:1 top gear for my setup (net 0.56:1 in double overdrive) but it seems like a big project to gain not much more overdrive.