The speedometer on my '85 K100RS read about 10% fast at any given speed. It can be adjusted by turning a potentiometer within the speedometer itself.
The pot is the small metal part in the photo just above the yellow plastic gear. Use a miniature screwdriver in the rectangular slot.
An approximate 90 degree turn to the right (clockwise) doubled the error to about 20% high. A turn to the left of about 40-45 degrees (as far as it would go in this case) seems to have exactly corrected the speedometer reading. ( A normal K100RS [2.81 final gear] characteristic is that when in 5th gear, both the speedo and tach needles remain exactly parallel to each other at all speeds.)
The pot is accessible only after separating the speedo from the instrument assembly. There is a single small black screw on the back of the speedo that attaches it to a plastic arm reaching from the center carrier. Remove that screw and the speedo can be moved forward to access the pot. When the instruments are out of the housing, a lot of very fragile pieces are exposed, so handle with care.
Photo courtesy of Brian Curry.