How to tell gear ratio from pinion gear stamp?

[mosel70]

I have a'70 C10 stepside. Appears the pinion gear stamp is NF GM86, hard to see in photo. I have photos of the other stamps on the differential exterior if that might help. Looking to swap trans from TH400 to 700R4 and need to know the gear ratio. Truck new had a M20 manual 4-speed. Thanks!

[mongocanfly]

Stamping should be on the ring gear ...not the pinion....your seeing numbers on the carrier...should be something like 41-10...or similar....like in the blue circle

[sweetk30]

thats NOT the pinion gear . its the ring gear carrier .

if you have the cover off for the pic's then roll the ring gear around and look on the end for the stamped in #'s that will give you the manufactured date and also the gear ration via the ring gear and pinion teeth count . if you cant find these #'s then count the ring gear teeth and divide by the pinion teeth count .

41-10 = 4.10
41-9 = 4.56

this is also the way it will be stamped in the face of the ring gear between the carrier and the end of the ring gear teeth .

[mongocanfly]

Dont forget that your tire dia plays into what ratio youll need...use a rpm calculator to determine whats best...http://www.csgnetwork.com/multirpmcalc.html