Where is the sensor for reading speed and mph

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 .

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

Erratic signals come from the speed sensors(all that I found). At the sensors by the tires/CV axles. When I tear them apart (the bearings) they have fine metal around them. The causes the erratic signals. As you said, they seem normal at most points. But you have to watch and see where(the signal) they jump. Could be 2 MPH or 20 MPH. Replace that wheel bearing.

change speed sensor on transmission.

There are also wheel speed sensors, so that may be a cause as well.

Speed sensor may be giving a false reading .

NO, IT'S NOT THE TRANSMISSION! SOUNDS LIKE A SPEEDOMETER PROBLEM! THAT WOULD BE ONE OF THREE THINGS! EITHER THE SPEEDOMETER, THE CABLE, OR THE SPEED SENSOR! THE SPEED SENSOR WILL USUALLY GIVE YOU A P0500 CODE ON AN OBD-II CODE READER! THE SPEED SENSOR IS ELECTRONIC SPEED CONTROL LOCATED ON THE TRANSMISSION! SOME HAVE A SPEEDOMETER CABLE HOOKED TO THEM, AND SOME JUST HAVE A 2 WIRE PLUG THAT HOOKS TO IT!

The speed sensor is malfunctioning on the transmission.I would not fix it as a result of the age of the car.Simply use a GPS for a more accurate speed

HI YOU COULD TRY CHECKING THE GEAR RATIO, AS THE SPEEDO DRIVE IS FITTED TO THE FRONT SPROCKET, ANY CHANGE IN THE MACHINES GEARING IE SMALLER OR LARGER SPROCKETS WILL HAVE AN EFFECT ON THE MACHINES SPEEDOMITER READING, AS WILL A CHANGE IN TYRE SIZE

IIRC, the speedo reads off of the sensor in the center section of the rearend. Also controls ABS. Hope this helps!