From under the car check to see if
The retainer which holds speedometer cable to the transmission has an "O" ring which is probably leaking... If this is where the problem is it is an easy fix......
Remove retainer, 1 bolt, and slowly pull on cable to remove retainer.
There should be an "O" ring, or a grove for one, near the outside of the retainer.
Replace and reinstall....clean area around and watch for leaks.
No, you don't have to remove transmission to replace this "O" ring.....