Z1OwnersClub GB

My GPz 1100 B speedo ( all in one dash) reads less than the actual speed.
Needdle moves, so does the counter, but it reads substantially less.
Anyone with a similar problem, or any suggestions for fixing it?
I have been looking for a new or used dash, but no result so far.
Thanks

Are you running standard wheel sizes, with correct profile tyres
Another thought is, are you running a kawasaki speedo drive, or has the bike been fitted with something from another make I believe the ratio's are different, or does it rely on an electronic pick up

Thanks for your hints, however the bike is standard in all wheels tyres and speedo drive, and the error is a lot. So, cant figure it out, I ll have to dismantle the whole lot and check.
My question was whether it was a known problem on this model.
Cheers