On my 1999 r1150gs the speedo always showed a fast reading according to the sat nav , over 10% at 70mph .
Checked it at work against a calibrated speedo and at a genuine 70 mph the bike was reading 78-79 mph.
Fitted a different speedo drive , for the life of me I can't remember which bike it came from it was either a 17" front wheel model r1150r or from a police spec r1150rt with a 18" front wheel but it brought the speedo accuracy to within 1 mph at 70 mph and spot on at 30mph .
On the two r1200gsa lc models and my old 2011 r1200gsa tc I've owned the electronic driven speedos were spot on