Speedometer driven gear issue

[msinc]

Originally Posted by ERA174

It appears you originally had a RH(right hand) 21 tooth driven gear, not sure why given your passenger side speedometer cable placement. The replacement 16 tooth driven gear you installed has LH(left hand) teeth, not a match as you discovered. I believe the correct replacement RH 16 tooth driven gear is Ford# CODZ-17271-A. You should check the teeth on the drive gear to make sure they have not been damaged by the LH 16 tooth driven gear. Shining a light in the hole of the tail shaft housing for the speedometer cable and rotating the output shaft should be sufficient for inspection. If the drive gear looks ok, replacing the driven gear with the part number I supplied should work. If the drive gear has significant damage the tail shaft housing will have to be removed to replace it. Depending on how severely the gears were damaged, you should probably also replace the transmission lube to eliminate as much of the plastic gear debris as possible.

What doesn't make sense in this case is that the drive gear appears to be the correct one. I can see it is pink, 6 tooth and undamaged. This means that the red 21 tooth driven gear with it's right hand gearing was incorrect from the start. However, it worked and read speed just fine, albeit wrong/slow by 10mph. A close look at the red gear shows slight damage but I still don't understand how it could have worked at all if the drive gear is in fact left hand. I can only see it with an inspection mirror. it is definitely pink and absolutely 6 tooth as it should be. It is my understanding that the 6 tooth pink gear is left hand. I have ordered both the drive and driven gears new and will find out for sure what's going on when I pull that tail housing. Thanks to everyone for the info!!!!! It is greatly appreciated!!!!