After taking apart my Volvo I cut off the sensor wires leading to the rear Diff as I assumed these would be for the ABS and/or traction light, but turns out it is also the speed sensor for the speedometer.

I assume that this is probably just a normal magnetic sensor with a small magnet on the gear inside the diff, so really I could probably use anyother speed sensor and mount a magnet on the propshaft or rear diff shafts?

Any ideas would be appreciated.

Thanks Mark

It will have been a hall effect sensor --- ie no magnets.

on a volvo it counts the teeth on the crown wheel.
uses the count for speed / abs / traction all very cunning.