I'm not sure how well calibrated those roadside reminders are. There are two on my way to work and both regularly warn me for 30 even when I'm stable at that speed and using the car's limiter.
They might also be tracking other vehicles.
Is there any reason why your speedo might be out - non standard wheels or tyres for example?
The easiest way to check it is using GPS on your phone - the Google Maps app can show speed. Try to check it over a good quarter mile on a straight road. Mine shows the speedo to be accurate at 30 but under reading by 2-3mph at 70.
The other way is on a motorway using the kilometre posts and a stopwatch while holding a steady speed. You'll need an accurate km/mile conversion factor and preferably an assistant to operate the stopwatch.
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