what pinion goes in the unit? I believe that is the only way to tell which unit is what. I have the 4:1 and the 2:1.
I believe the 2:1 uses the 21 tooth pinion.
http://store.rc4wd.com/Pinion-Gear-f...it_p_1919.html
And the 4:1 and 3:1 use a 12 tooth pinion
http://store.rc4wd.com/Pinion-Gear-f...it_p_1920.html
Not sure how to tell the 4 and 3 apart. So if you have the 21 or 12 pinion, you'll at least narrow it down a bit. Heck they make a 1:2 as well! That takes a 15t pinion.
Also, these units were not made to take a low turn motor. The internal gears wont hold up for long.