Try using your original speedometer gear
I recently had the two-speed Ford-OMatic transmission in my 1956 Fairlane rebuilt. The speedometer gear (which has 20 teeth) worked fine before the transmission was rebuilt. I purchased new nylon gears from two sources. Their gear grooves are not the same as my original part. Holding my original gear by the stem, the grooves at the top tilt slightly right of 12 o’clock. The new nylon gears tilt slightly left toward 11 o’clock. I’ve telephoned two parts sources, but theirs tilt slightly left of 12 o’clock too. I installed one of the nylon gears but the speedometer would not function at all. I believe my transmission is the original one for my car. Can you or any of your readers help me?
If, as you state, the 20-tooth speedometer gear that you removed from the transmission was working fine before the rebuild I don’t understand why you don’t just put that gear back in. Without seeing the parts you’re dealing with I can only speculate but it seems likely that the pitch of the teeth of the new nylon driven gears you have purchased don’t correlate to the pitch of the teeth of the transmission’s internal drive gear.
All of the dimensional characteristics of the speedometer drive and driven gears, including teeth spacing, angle and size, need to match each other. If the driven gear you removed cannot be reused for any reason and the readily available replacement gears don’t match the drive gear inside your transmission, you will have to find a driven gear that matches the drive gear you have or change the drive gear to match the readily available driven gears. If you can simply use the original driven gear that was removed I suggest that you enlist the help of the person who rebuilt the transmission for you.