Tachometer
The tachometer is an analog gauge that is located in the instrument panel cluster (IPC). The tachometer is operated by the IPC with software control. The VCM/PCM sends Tach pulses to the IPC on a discrete circuit at the rate of 2 pulses per engine revolution with the engine running. The IPC will interpret the tach pulses in order to appropriately display the engine RPM on the tachometer.
Speedometer
The speedometer is an analog gauge that is located in the IPC. The speedometer is operated by the IPC with software control. The VCM/PCM sends vehicle speed pulses to the IPC on a discrete circuit. The PCM/VCM sends a Class 2 message to the IPC when the engine is running. The IPC will then interpret the speed pulses in order to appropriately display the vehicle speed on the speedometer. An electric speedometer drive is standard equipment on all models. Integrated circuits control the air core speedometer and the electric odometer. Lighting is provided by a removable bulb and socket assemblies which insert into the back of the instrument cluster.
The speedometer system consists of the following components:
• The instrument cluster
• The vehicle speed sensor (VSS)
• The PCM/ VCM and related wiring
The one-piece instrument cluster mounts in front of the driver in the instrument panel. Replace the instrument cluster if the diagnosis indicates that the instrument cluster is malfunctioning. Refer to Instrument Cluster Replacement . Repairs to the instrument cluster can only be performed by an authorized instrument cluster service center. The PCM/VCM converts the analog output form the vehicle speed sensor to a digital signal containing 6,400 pulses per kilometer (4,000 pulses per mile) for the instrument cluster. The PCM/VCM is matched to the final drive and tire size of each vehicle. If the final drive or the tire size is changed for any reason, update the PCM/VCM in order to match the new final drive or tire size. If the PCM/VCM is not updated, the signal will be inaccurate for correct speedometer/odometer operation. An incorrect signal can affect antilock brakes and cruise control systems. The VSS is a permanent magnet signal generator that spends an analog signal proportional to the propeller shaft speed to the PCM/VCM. The VSS mounts to the transmission tailshaft extension housing on a two-wheel drive models and the transfer case output shaft housing on the four-wheel drive models.

Could be the instrument cluster or speed sensor , best bet have a qualified repair shop check it

You might have to replace the speedometer cable