1998 Pontiac Firebird Speedometer

Try checking your speedomter cable. To do this you need to get under the car by the transmission. Youll see the cable running into the tranny right behind the tranny mount on the drivers side. Disconnect it and seed if you can spin the cable inside the sheath. It should move fairly easy. If not, you may have a bad cable which could cause a false speed reading. If it is bad. Before replacing. Id spray the inner cable with some wd-40 and see if it loosens it up. If not, it needs to be replaced. Good luck :)