Gps is accurate if it can 'see' the satellite, but can struggle to keep up if driving under trees, near tall buildings or low cloud
But it is far more accurate than the imaginary number that the speedo says
Calibrated speedos aren't fitted to all Police cars, so Mr Plod has to be quite sure that you are speeding before pulling you over. So a traffic car with calibrated kit will pull you if you are consistently doing 10% over the limit with evidence to prove it, whereas a panda car might pull you at 15+%, and then only to give you a yelling at, as they cannot 'prove' that you were speeding
You can be arrested without evidence, ie on suspicion, but can only be charged with actual evidence
Back to your earlier question though
I suspect the 3.2 auto will be nearer the 8 second mark