EBAY type combo speedo tacs

[chiz]

Been looking at the plethora of combo speedos and tachs on EBAY and notice the images almost always show a threaded shaft with a nut with something slightly sticking one end of the shaft, wires emerge from the other end.  I assume this is the speed sensor or is it the tach sensor.  Anyway regardless I would like to know how and where this device is mounted as the sales pitch does not say. Anyone here hazzard a guess?
   Chiz

https://www.ebay.com/itm/Universal-Motorcycle-Digital-LCD-Speedometer-Odometer-Tachometer-Gauge-12000rpm-/163438905801

[Firehawk068]

What they are showing appears to be a "Proximity Sensor"  (we use them on a lot of equipment at work)
It would have to be mounted to some sort of bracket near the wheel.
You would then mount a couple magnets at various points on the wheel or brake rotor.
The sensor picks up the "Current" that is generated when the magnet passes by the sensor coil.
This tells the logic circuit inside the speedometer how fast the wheel is spinning.

[chiz]

OK thanks mate