This is my first post, had my 88 K100 for about 500 miles now and loving it. Where have you been all my life?
From what I have been reading excessive speedo error appears to be the norm. My experience as best that I recall is that on my bike I am turning about 4000 rpm's and the speedo is showing 70 mph and I am being passed on a regular basis. Is there an expected mph reading at a given rpm that I can compare to? I realize that engine tune may affect the accuracy but on a comparable bike with the same final drive ratio as mine I would expect similar mph/rpm ratio.
First of all, don't ride/drive at the speed limit, ride at what traffic is going. It's much safer.
In general, it is exactly 10% fast. 80mph indicated is 72 mph actual. It depends on your bike's setup/FD ratio, but with a 3.2:1 final drive on a K75, 6000rpm is 80mph actual.
There is an app, Karamba, somewhere here or on flyingbrick.de you can use to calibrate.