[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.