[Big Dave]

The GV overdrive is a single planetary spur gear setup. Expect a 3% loss in driveline efficiency, but this will be more than offset by the lower engine RPM resulting in lower engine frictional HP.

What concerns me about t_vago's setup is reduced cooling in the automatic transmission due to the big overdrive ratio.

I had a GV on a 1983 GMC K2500 with a TH400 tranny and the wimpy old 6.2 engine. I destroyed not one but two TH400s in front of the GV. Once I raked up the parts from the Interstate, I took them to a transmission guy and he said excessive heat buildup.