Check the vehicle speed sensor, located in the right side of the transmission extension housing. This sensor reports the vehicle speed to the pcm, but sometimes the signal is validated (compaired) to the front wheel speed sensor. If the rear sensor is failing (or failed) you will get an erronious reading from the speedometer. Check connection to this ssensor for corrosion as well since the vehicle has sat for a period of time.
Just a suggestion but have seen this myself on many occasions
Tuesday, July 28th, 2009 AT 10:55 PM