[dap930]

The electronic speedometer works by receiving 8 pulses per wheel revolution from the reed switch (puck) mounted to the outside of the transmission. There is a disk inside the transmission that has 8 magnets mounted to it. When a magnet is aligned with the red switch, magnetic flux causes its contacts make which provides a ground input to the speedometer. Inside the speedometer there is a pull up circuit that provides a 12 VDC signal input that gets interrupted to 0 VDC every time the reed switch provides a ground. So, this creates a square wave signal to the speedometer and odometer drive circuits. A speedometer calibrator intercepts the pulse signal from the reed switch and alters the pulse rate by a percentage to increase or decrease the pulse rate. For most Porsche speedometers, the pulse rate needs reduced because most read 5 to 7 MPH too fast. But you must check the speedometer readout a different speeds, because the speedometer may not be linear. My speedometer reads dead on at 60 - 70 MPH, but still reads a little fast at lower speeds after setting up a speedometer calibrator.