Turn lights on - speedo goes up to 160mph

[paulharrison123]

Hi All, I have a PH2 172 with a very annoying problem, usually everything is fine, however if It is wet (and sometimes in the dry to be honest) and I turn my headlights on (xenons) my Speedo goes up to and fluctuates at 160mph (drops to 90, goes back up etc, not relative to speed)

Im confuzzled and am presuming a bad earth / faulty pulse sensor or something?

Any help will be massivley appreciated as its making the mileage go up as if I really was doing 160mph.... ​