I could open the meter gauge and see if there is any adjustment there.You know, virtually ALL speedometers have SOME method of calibration.
It sure would be easier (and more reliable) to simply calibrate the speedometer itself, than try to kludge in some makeshift circuit.
It may be simply that water or other contamination has entered the speedo housing, and is causing a lower than normal resistance in a portion of the circuit. Careful cleaning the board with 91% isopropyl alcohol may simply make your problem go away.
But, FYI, this error is by design.
Pretty much all motor bikes will read about 5-10% high.
It will depend by manufacturer, but it is engineered that way.
The meant for it to be off.
Unlike with cars, where they are pretty close to accurate.