Hey 64 & 66 C10. Try Googleing 350TH speedometer gears. You'll come across a couple of sites that give the color and number of teeth of the speedo drive & driven gears. The driven gear, ( the one that is attached to your speedo cable) is quite easy to change out. The drive gear, that is internal will require removing the tail end off the transmission, if you need to change it out.
Hope this helps.
