legacycentral bbs

My tranny has been leaking a little from the pan area. Until I can get it properly fixed I have been simply keeping an eye on the fluid level (It's a pretty slow leak) and just topping it off when nessacary. However it is really hard to tell from the dipstick the actual level so I have been guestimating. Yesterday I was planning on driving for a trip and topped off the fluid just to be safe. I notced that about 50 miles into the trip the speedo seemed to be stuck at 75 and would very slowely react to any changes in speed. The cable seems to be in properly. It works pretty well at low speeds but is still a little sluggish. I know the cable connects to the front diff area. Could I have over filled the fluid a bit and its been causing problems?

Seems to work (although sluggishly) up until 75 then it stops going any further. Cable issue or gauge issue? I think I am going to try draining the tranny fluid and replacing it to rule that out.