Sorry I cant help you with that one.
It could be the settings in the system, you may want to check that first, and make sure its setup to drive to speed limits. Like everything else there are varying levels of assistance which can be individualised.
Its not something I would use anyway. Every speedo is set a few miles less than the actual speed. Manufacturers build an error of 2 or 3 mph. Such that with cruise control i set at 32mph in town and 74 on the motorway.
Easy to check, use google maps or Waze for navigation they show your true speed rather than the speedometer.
I would go through the settings menu first, just to ensure it is set correctly. Some of the menus you have to swipe up to see the complete options.
Good luck, you will get there, its just a pain.