Possible Causes

incorrect tire size
PCM
instrument cluster

INACCURATE SPEEDOMETER READING PINPOINT TEST REFERENCE

PINPOINT TEST 4		INACCURATE SPEEDOMETER READING
STEP	INSPECTION		ACTION
1	CHECK THE TIRES FOR CORRECT SIZE Check for factory recommended tire size. Are the tires the correct size?	Yes	Go to next step.
1		No	Install new tires as necessary. Test the system for normal operation.
2	CHECK THE INSTRUMENT CLUSTER PID Connect the diagnostic tool. Key ON. Enter the following diagnostic mode on the diagnostic tool: Instrument Cluster PID Select and monitor the instrument cluster English/metric PID. Does the PID read KPH for a KPH instrument cluster, or MPH for a MPH instrument cluster?	Yes	If the speedometer operates correctly but is inaccurate, go to step 4. If the speedometer is inoperative, see PINPOINT TEST 5: SPEEDOMETER/ODOMETER IS INOPERATIVE .
2		No	Go to next step.
3	CHECK FOR CORRECT INSTRUMENT CLUSTER OPERATION Disconnect all instrument cluster connectors. Check for: corrosion pushed-out pins Connect all instrument cluster connectors and make sure they seat correctly. Operate the system and verify the concern is still present. Is the concern still present?	Yes	Install a new instrument cluster. See INSTRUMENT CLUSTER . Test the system for normal operation.
3		No	The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.
4	CHECK FOR CORRECT PCM OPERATION Disconnect the PCM connector. Check for: corrosion pushed-out pins Connect the PCM connector and make sure it seats correctly. Operate the system and verify the concern is still present. Is the concern still present?	Yes	Install a new PCM. See POWERTRAIN CONTROL MODULE (PCM) REMOVAL / INSTALLATION . Test the system for normal operation.
4		No	The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.