When does it read 100 mph ? sitting still , all the time etc.... ? The speedometer is controlled by a step motor inside the instrument cluster , step motor is controlled by a microprocessor inside instrument cluster , which get's the speed info from the PCM - engine computer over the vehicle class 2 serial data network . The VSS - vehicle speed sensor is a input to the PCM , this is what the PCM uses to calculate the vehicle speed .
Speedometer
The IPC displays the vehicle speed based on the vehicle speed signal from the PCM. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit from the PCM in order to calculate the vehicle speed. The speedometer defaults to 0 km/h (0 mph) if a malfunction in the vehicle speed signal circuit exists.
Having the vehicle hooked up to factory scan tool can take all the guess work out of a diagnoses like this .
SOURCE: Speedometer Not Reliable
This happen to me too. Had to get it replaced under my exstended warrenty that cost me the $200 deductable but would of coast over $400 if this happened later.
SOURCE: 1994 Mustang speedo reads fast at speeds over 30 mph
someone has modified. the speedometer cable has to be taken loose from the tranny and put back in. the only way it can be that wrong.
SOURCE: Speedo not working
With the engine off,and ground battery terminal off.
Use an ohm meter the trace the VSS connector back to the ECU.
It should be the same color code.
Follow the wire and look for cut(s) along the way.
SOURCE: chevy silverado 1500 engine light come on at 65
Autozone will check the codes for free .Itsounds like a secondary ignition problem possible plug wires :)
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