I used to have a GMC K-2500 with a 6.2 NA (later had a Banks turbo) that had a GV overdrive. Some observations:
It flat-out improved my MPG
from 20 to 22 MPG
. The 6.2 was wimpy but economical.
The GV is a well-made piece of machinery. The electrical lines are exposed so you must protect them or the thing will drive you nuts. Mechanically...bulletproof. It uses planetary spur gears for reduction and has a slight spur gear whine when engaged.
The overdrive ratio is 0.8:1. This isn't quite as deep as the 0.72 built into a ZF-6-650.
I will flat guarantee you it will destroy automatics like Godzilla destroyed Tokyo. Spectacular failures...scattering parts and ATF
down the road. Almost always in Interstate driving. I never though I'd live to see the day a TH400 could be broken but the combo of 6.2 diesel (one of the wimpiest ever) and the GV trashed one every 12,000 miles or so. Even an Allison torque converter didn't help. Ford, Dodge, or even the Allison 1000 behind Duramaxes will be wrecked by these units. Maybe a Brian or a Suncoast can make a special that will stand up, but I gotta see it to believe it. The GV slows down engine RPM at a given road speed and the automatic cannot pump enough ATF
to the cooler and they get hot and scatter parts.
A GV works fine with a manual if you can live with two OD
Reducing engine RPM at a given road speed does improve mileage by reducing engine frictional HP. This is why my 3.08 gears work. I've done a bunch of research on this and I'll post another thread on the subject.
Optimal RPM for MPG
? It will undoubtedly vary from engine to engine but I'm of theb opinion that 1200 RPM @70MPH is about optimal. Five feet forward per engine revolution. My 7.3 is an unmitigated torque monster and can handle this. I'll be posting more on the subject.