Not sure but if it is flicking between KPH and MPH it sounds like it could be either a fault with the speedo or the wiring to the speedo. It could also be something as simple as the speedo losing its settings due to the battery being disconnected at some point.
I wouldn't have thought that the sender would cause a fault which changes between MPH or KPH but it is worth checking that it works, Again not knowing the type of sender or speedo, I'd test it as follows:-
Connect a multimeter to the sender (see next two paragraphs for what you are using the multimeter to do) Jack up the back of the quad, and rotate the wheels (using you hands, you don't need to be in gear and ignition should be turned off) this should result in a fluctuating reading on the multimeter.
Try taking the connector of the sender and see how many wires there are, if there are 3 one needs connecting to a live feed and using a multimeter between the other two connectors you would look for a hig and low voltage being generated (0V for the low and a high equal to the voltage on the live feed)
if there are 2 wires then it is working based a circuit opening / closing using a Multimeter you should be able to test the sender by looking for the circuit opening /closing. Looking for high then low resistance, open=infinitely high resistance, closed=very low if not zero resistance.
If you prove the sender is working you can check the wiring, again using a Multimeter first check that if you have a sender with 3 wires one of them has a voltage on it (you will probably need ignition on to do this, so if working near a moving and HOT engine be very very careful!!!)
I hope this helps
Simon
