The gear on the output shaft is called the driving gear and the one on the sender is called the driven gear.
The driving gear can get worn out (have experienced this).
If you pull the sender out and feel the driving gear with your finger by moving the finger across the gear it should be flat. If worn out will have a noticeable inwards curve.
To correctly test the sender you really need an oscilloscope (see the pulses coming out as it spins) and to test the speedo you need a pulse generator (input pulses to move the speedo needle). Not many have that type of gear available, but below is a simple test...
Check continuity of all the wires between the sender and the back of the cluster.
At the sender end, the red wire should have 8v from the cluster, the black is ground (chassis/battery -ve) and the white wire is the pulses into the speedo.
A quick test if you have a multimeter:-
With the sender removed from the trans but plugged into the loom... ignition On.
Set meter, on DC 12v range, stick the red probe on the red (8v) wire (or +12v battery) and the black probe on the white wire. Turn the sender slowly and the voltage should fluctuate. One of the old needle type meter would be better but if you turn the sender slowly it should go to 8v then 0v then 8v etc.
If you stick a drill onto the driven cog and run it then the needle on the cluster should indicate (make sure you hold the sender firmly if doing this test).
Oh, and the XE sender is same as XD. XF onwards will fit but as mentioned above has a different number of pulses per revolution output. As I recall (but could be wrong) the pinout is different between the XD/XE and XF.
