The problem with using satnav to identify speed limits is that in-built systems are usually out of date and don't reflect changes in the limits - even online satnavs like Google Maps or Waze aren't totally accurate.Mine showed a 40 mph limit on a local busy town road, and it has never ever been 40mph. When I updated the navigation software to the Spring 2025 release it corrected that error and now shows 30mph, and goes red if I exceed it by 1 mph or more. It uses GPS to know where it is, but its nav software to show speed limits for where it is located.
The mk1 human eyeball and brain is still the best there is - but very under-utilised.