The speedometer seems to be way off, it was not working, found that the plastic gear in the transmission was stripped, replaced it with one I had 20t, was wondering how much each tooth more or less would make a difference on the speedometer, seems to reading way to fast?

As you have found out, You need to replace your broken one with one with the same # of teeth

Just bought the truck and speedometer was not working changed from19t to 20t only one I had not much change but speedometer seems to be about 10mph off.

Not knowing if it was off before, It will be hard to tell what the 1 extra tooth doe's.