I guess you raise a few issues - as said by others, the speedo in most cars reads greater than actual (speed) - however, my experience is that this is not uniform throughout the speed range - for example my 74 plate euro box Suzuki needs to be run at 75mph to register 70 mph on the GPS system. But, at 30 MPH, probably because that is a common speed limit in the UK (and where most people end up being fined), there is zilch difference between the GPS and speedo.
2nd point is that in the UK , since July 2024, all new cars, as in alignment with the EU, are fitted with TSR - road traffic sign recognition - which, in my case, sends a beeping noise when the speed limit is exceeded (for that part of the road). The TSR is activated from the speedo NOT the GPS. What does this mean in practice ?...well, one is adhering to the speed limit of 70mph in the outer lane of a motorway (speedo reading) and as the cue of cars behind you soon let you know that you are actually only doing 65mph...please get out of the way !! Of course, the natural thing to do is go faster to 75 mph - however the incessant bonging noise coming from the dashboard is so bad you reduce speed and revert to the inner lane.
To turn off the TSR is brand and maybe model specific but there has to be ( as told to me by my Suzuki dealer) that there has to be 2 independent switches to be "enabled" to turn this dammed thing off. If one can achieve this, adding insult to injury then occurs because on turning off the engine, the system resets to default mode and one has to go through the cancellation procedure all over again !
Enough said?
