Your car has the A 604 transmission also called the 41TE,and it uses an input and output speed sensor mounted on the transmission.These speed sensors feed information to the Transmission Control Module (TCM) one of the vehicle's computers, which calculates vehicle speed,and sends a speed signal to the speedometer in the dash.It is a totally electric speedometer and odometer,no cable drive like old cars.So the first step would be to scan the vehicle's computers for diagnostic trouble codes,as a malfunctioning speed sensor can cause your symptoms,sometimes accompanied by no upshifting,or erratic shifting.All the components involved in the system have to be deemed good,or repaired if a component is found defective.With the scan tool,there is also a data stream available for viewing the systems while the engine is running and the car driving,to look for malfunctions.Within the data stream,i can look at the speed sensors individual outputs,and see what the computer is seeing as far as inputs and outputs.If the speed sensors check out ok,and are functioning properly,then the problem would likely be the instrument cluster,including the speedometer in the dash.But other components have to be verified good before drawing conclusions.