Another gearing idea...

[rm25x]

Was trying to think of other spur/pinion combos for a lower ratio without using a gear reduction...

84/12 48p stock gear: 7:1
16/40 axle ratio: x 2.5:1
Total stock ratio: = 17.5:1

72/6 32p gear: 12:1
16/40 axle ratio: x 2.5:1
Total new ratio: = 30:1

Used a Traxxas nitro Rustler/Tmaxx spur, RC4wd pinion. Just not sure if the spur would fit between the frame rails, I already know that it would rub the lower skid (at least I would have to assume so)

If you wanted to use a gear reduction...

If you ran a GD-600 at 3.8:1 ratio, with stock gearing...
84/12 48p stock gear: x 7:1
16/40 axle ratio: x 2.5:1
Total GD-600 ratio: = 67.45:1

If you ran a GD-600 at 3.8:1 ratio, with new gearing...
72/6 32p gear: x 12:1
16/40 axle ratio: x 2.5:1
Total new ratio: = 114:1

I think I may have to try it out

[OGmicromonster]

Sounds like good possibilities. I've been running my 3.4:1 gear reduction with a 22 pinion and 93 spur. It works pretty good. Although the 93 spur did fit between the side plates, it did rub on the lower skid. So to fix it I just used two skids the first one with a slot dremeled into it for the spur gear to clear and the second to cover the hole from the bottom.

[rm25x]

Ended up with a GD-600 to get the gearing I needed. Ran the 12t pinion on the GD, and the pinion the GD came with to the 84t spur. Was just about right.