100 %with you on that! But I do know what the difference between GPS speed and speedometer is in my cars. I just do not want an extra app tracking my life just because I like to have a HUD.I prefer the speed indication of the GPS of my phone. This is almost 100% correct. The speed indicator of the car itself (through the OBD) has a fault margin of around 5%.
Because I like to drive as fast as possible within the allowed speed limit + fault margin of the police (when it's safe to do so of course).
In Belgium, on the motorways/highway speed is limited to 120 km/h. It is known (appears sometimes in the press) that Police also has some fault tolerance. On the motorways you get a fine when you drive 129 km/h or higher!
Therefore I drive 127 or 128 km/h (with cruise control)!
(I know also the fault margins for other speed limits).
If it needs a phone to work I will not buy one.