520,
This page may help you:
TCI Speedometer Gears and Accessories | JEGS
The driven gear is most likely the one you need to replace, and there's a selection shown near the top along with ways to estimate which one may work for you.
It's on the speedometer cable end outside the trans and is easy to replace. Some fluid may try to escape so take some care...
Count the teeth on the existing gear, then select a new one that is closest to the number of teeth you decide you need.
Near the bottom there's a clip you may need.
Tom