Again, most likely the plastic dust cap. My speedometer acted like yours, but it was fine again after taking the left front wheel off and and putting it back on (I found this by coincidence, because I took the wheel off for another reason.)
On another note, my speedometer is off by 15% (shows 115 at 100 on GPS). I've been doing some research, but it is next to impossible to get this corrected.The first question however is, whether your odometer is off too, and by how much. Having lived in Europe, I installed a metric version of the instrument, which makes it extra difficult to check with mile markers.If the odometer is off too, there is not much you can do. If the odometer is accurate though, there should be a way to adjust the speedometer, either by regaussing the unit, or install a different coil spring.Does anybody have any experience with that?