I had a similar problem with my 82 1100, jacked up the front end, spun the wheel and the flat metal tab seemed to turn ok. Tried lubing the cable, replaced the cable. Nothing seemed to help, finally tried spraying some silicone spray into the speedo where the cable screws on, that worked for a couple of weeks, so I sprayed it again and over did it, the insides got too slippery and the needle quit going up with the speed, ended up getting a new speedo off ebay, worked great. Got a new bike out of the deal with 0 miles. Good luck with yours.
Lloyd and Judy
08 GL1800 ABS with a Hannigan GTL sidecar