Kz900 speedo too fast

The speedo/odo on my 76 kz900 seems to be fast by about 10% or so. Running correct tire on stock wheel. This one has me perplexed....

It's not uncommon for a motorcycle speedometer to be incorrect. I have seen it reported even on newer models. On an old bike the problem may be the magnets inside the speedometer weakening over time. Z-Resto restores early KZ meters & one of the things they do is replace the magnets.

I have a speedometer that is the opposite, it's extraordinarily lazy. Thanks Martin, for the Z-Resto suggestion!

Had a chance to check it against a gps today. It's WAY off. Reads 60 when actual speed is 50, reads 82 when going 70. Anyone got any ideas?