[Big Dave]

I have the GV overdrive in my F-350 although with a stick shift. The GV’s 0.78:1 ratio combined with the ZF’s 0.71 ratio gives me an 0.554:1 overall ratio.

The double overdrive does improve MPG by about 1.25 MPG for me. With OEM-size rubber, I am turning 1310 RPM at 70 MPH. 1125 at 60 MPH. My big diesel turns this with insouciant ease. I kinda have to use the GV because there are no commercially available alternatives without sacrificing the capabilities of my truck, which I am not prepared to do.

Couple of caveats about a GV setup. Have your automatic worked on by a really good performance transmission builder. Tell him about the GV.You will be slowing the torque converter down a lot and heat is gonna be a problem.

Secondly, GVs are very heavy-duty pieces of machinery (originally built for Bedford military deuce-and-a-halfs) and are priced accordingly. It would be cheaper for t_vago to get a T-56 and do the “sex change operation” to convert his truck to a manual and he’s still have enough money left over to pay bondo to make him a very slick über-quality aerolid. Then he’d have a 0.5:1 top overdrive and only have to overcome the losses of one gear mesh. The T-56 gets around the frailties of the automatic altogether.

The optimum driveline would be a manual transmission with a straight-through (1:1) top gear and an appropriate gear ratio (about a 1.5:1). Numerically low ratios are getting hard to find. 3.08s and occasionally 2.73s are about it (and thise take some looking) unless you have an outstanding junk yard at your disposal. The OEMs are under governmental pressure over NOx. High load generates more NOx than light load. NOx emission is fairly RPM-neutral. So the OEMs tend to use 3.73s to keep load down and RPM up.

To me the optimum MPG driveline for my truck (considerably heavier than t_vago’s truck, obviously) is to have a custom rear axle made with a 13:20 (1.56:1) ring and pinion and ditch my ZF6-650 and GV for a Spicer seven-gear with 1:1 top gear.