Hello, and welcome to our forum community.
The ST 1100 speedometer is an old-fashioned mechanical system. It uses a gear assembly on the front wheel to turn the cable, which then drives the indicator up in the instrument panel.
There are at least 3 possible sources of problems that you should investigate:
1) The drive gear on the front wheel - it is possible that this drive gear is not working properly, perhaps due to wear or damage. To inspect it, you need to remove the front wheel (not a particularly difficult task).
2) The cable - it may not be correctly fitting into the drive gear at the bottom or the speedometer up at the top, or it may require lubrication to allow it to turn freely. It's also possible (not likely, but possible) that it may have broken at either end or somewhere in the middle.
3) The indicator (speedometer itself) - this should be a pretty trouble-free device, but it is possible that there is something wrong with it. I think the most common problem is that the cable is not properly inserted into the indicator.
Hope this helps.
Michael