I have thought that I would like to do the ZF-6 swap someday. Along with it going to 3.55 gears. My current situation is the ZF-5 with 4.10 gears. The way I see it, there is not much advantage in my situation if I should only do the ZF-6. With the 6 and 4.10's I don't gain any more OD, which is the biggest problem with my gearing now. I have excessive RPM at highway speed, limited top speed, and reduced fuel economy. If I leave the 5 and go to 3:55's I will be sacrificing towing ability. The way I look at the difference between the ZF-5 and ZF-6 is that the gears (the way they are labeled on the shift knob) there is an extra gear between L and 4(DD = 1:1) in the ZF-6 compared to the gears between 1 and 4(DD = 1:1) in the ZF-5. In both the top 2 gears are pretty much the same DD and OD. In the ZF-6 the gaps in the ratios are closer from L-1-2-3-4 than in the ZF-5's 1-2-3-4. The way I look at it there is an extra gear in the middle to close the gap. I think it makes the ZF-6 more suited for towing while allowing one to still have a rear diff. ratio suitable for highway cruising. In this way I think you could have a well rounded gearing combo to achieve good towing capability, fuel economy (empty/loaded), better cruising RPM, and max top speed. I plan on doing it with a 3.55 gear swap also.
FWIW, I went and looked it up...here are the gear ratios for the S5-42 and for the S6-650:
S5-42: 1 - 4.14 2 - 2.37 3 - 1.42 4 - 1.00 5 - 0.77 R - 3.79
S6-650: L - 5.79 1 - 3.31 2 - 2.10 3 - 1.31 4 - 1.00 5 - 0.72 R - 5.23
I hope that came out properly; trying to put the equivalent gears on the two transmissions right above one another. In any event...it seems to me that 2nd and 3rd are pretty close between the two. First is much higher on the ZF6; OTOH that's what the granny gear's for. I didn't realize that the 6 had a taller OD gear. The lower reverse on the ZF6 would be a nice thing.
BTW, one other factor to keep in mind...if you have a 4x4, IIRC the transfer case mount is considerably different and the SuperDuty transfer case drops on the opposite side. If you could find a ZF6 adapter for a U.S. Gear unit (IIRC they do exist, and then you'd have 12 forward gears
but, especially after U.S. Gear withdrew all support, they're probably more rare than hens' teeth), you could use that as an intermediary...
I really don't know...anything is possible with enough engineering and $$ thrown at it, and I'd be lying if I said that I didn't think that a ZF6 would be a sweet addition (especially with an OD unit hung behind it)....but, even without taking into consideration how much money would be spent purchasing the transmission itself, I can see this going up on the wrong side of $2K in adapters and customization right quick, especially on a 4x4. Given that it doesn't seem to me that you're really gaining
that much, it seems to me that that money could be better-spent elsewhere on the truck.
