Dually gearing

[Scottcycle]

Hey guys. I'm new here and have a few questions.  First off i have a 89 3+3 dually that i am installing a 700r4 into. It is just a driver so i don't need too much beef behind the motor which is stock. I need to swap spedo to a mechanical unit as that is what the truck is. I understand there are many different sizes of gears for the out put shaft and need help figuring out what ones i need. Is there a way to find out rear dif gearing without opening it up? Does anyone know where i could get spedo gears? Any help would be appreciated. Thx.

[zieg85]

If the rear end has not been changed and the rpo sticker is still in the glove box I'd say yes the info is available,

[Captkaos]

The truck should have a electronic speedo, if so just get the adapter for the 700R4.