IMHO that is not the best route to go. Nothing wrong with having AWD and the existing Granny Gear (labeled as "First Gear")in the existing system. When in true 4x4 "mode" the ratios available (1st thru 5th) pretty much cover everything someone needs when off road. It covers from what would be equivalent to bottom gear of low range in a typical 4wd Tcase via granny/1st gear all the way to an equivalent to high gear in 4wd High; via the 1.2:1 in
5th gear. Added benefit... is when in the existing tranny's 4wd mode you don't have to do anything to reach those ranges... no Tcase buttons or levers to activate when you need a gear that would fall in low range or gears that would fall in high range..... it is all automatic.
What is needed is more alternative ratios above 5th gear in the existing tranny and this may very well be possible via programming changes to the existing tranny computer. Thus, what you really need is someone who has permission from Allison to rewrite the tranny operating software to program the existing tranny to use other physical gear combinations that are already possible via different combinations of the existing physical gears there now. Or someone who can build a second computer to hijack the existing computer and trick it to do these things and not touch Allison proprietary programming. (or a hacker?)
It is also very very possible other physical gears can be put in the tranny ifffffffff software could be written to handle them. Way I heard it there is several of physical gears available that would fit into this tranny cause it is based on a widely used Allison 3000 model tranny which is; via software only and/or physically geared different plus different software, set up for use in various applications) This is straight info from a Tranny rebuilder extremely familiar with these transmissions. He does not know of an Allison programmer though... I asked him. But he has never looked for one either!!!
Tranny ratios for 3700sp and probably for version just before that are
- First 6.93 : 1 .......... Granny only available in 4x4 mode
- Second 4.18 : 1
- Third 2.24 : 1
- Fourth 1.69 : 1
- Fifth 1.20 : 1
- Sixth 0.90 : 1 (not available in 4x4 mode)
- 7th 0.78 : 1 (not available in 4x4 mode)
NOTE:
Italics is all gears available in highway AWD mode.
Example an 8th gear that is 0.5x would be sweet