Check your speedo cable,if it’s starting to fray,it will give inaccurate readings.
Ride through an illuminated speed sign a few times at various speeds,to get a rough idea of how far out your speedo is.
If you are still having problems might be an idea to check you have the corrrect speedo for a supa5. All circular MZ speedos will fit but the internal gearing on the ETZs can be different. The correct speedo will have a ratio of 1.6 stamped on the base somewhere.