Seems like the fewer teeth drives have too much of a squared off larger tooth surface. The drive inside the gearbox feels like it is more of a fine worm type drive. Perhaps the drive gears are specific to batch years of specific SM420's or trucks. I could not even get the 11 tooth to register a movement in the speedo and when i pulled it back out it appears the teeth are getting chewed on a bit due to a non mesh.. Just seems like it is not a clean drive fit.
