Howdy,
I think you may be getting yourself confused between a tacho and a speedo.

If the speedo picked pulses up from the distributor, the relationship between the number of pulses per kilometer would change dependant on what gear you were in.

The speedo I fitted is driven by a hall effects sensor. the sensor is mounted so it can pick up the pulses created by one of the bolts on the uni joint on the tail shaft as it leaves the gear box.

The sensor was supplied with the speedo. We then used a GPS to calibrate the speedo. (pulse per kilometer).
It can also be calibrated over a known distance. the manual that comes with the speedo should explain how.

Cheers,
Geoff