Welcome to TGO. Posting the year, make, model, etc. helps members better understand your problem. For example, Camaros were cable driven 82-89 and electronic thereafter. Birds were cable driven from 82-85 (IIRC) and electronic thereafter. So if your car is cable driven, check the following link:
http://www.thirdgen.org/techboard/in...eedo-woes.html (Reasons for Mechanical Speedo Woes)
If electronic, I'd suggest that your VSS, located in the tail shaft of your trans, is the culprit.
Oh, a search on TGO will often provide the info you're seeking.
JamesC
