GPS speed is an interesting debate, but to me it has become esoteric and technical while ignoring ‘why’ are we worried about it. When all is written, GPS computed speed is imperfect yet accurate enough for a purpose.
A speed display is to avoid an infringement. How many drivers on any road conclude ‘I should not be going over 60 kph’ here because it would be unsafe as compared to ‘I should not be going this fast here and need to slow down.’ Setting and enforcing what is usually an arbitrary limit is seen by some to be ‘part of the toolkit’ to reduce the road toll; it is certainly a revenue raiser; but if one steps back to look at what constitutes safe driving I propose GPS for speed measurement has as much merit as a mechanical speedo, or its electronically simulated counterpart, excepting it is more problematic to accept it for the purpose of issuing infringements, although that could be addressed by rational drafting of related legislation.
What is the purpose of a limit and which speed is more accurate? In the context of road safety why would 1 or even 5 kph matter? Is it because speed kills and that is an absolute above all other concerns? I suspect there will be at least one rebuttal that a crash at say X+5 kph will be more serious than one at only X kph, and nobody will change that or my alternative mindset. But avoiding a crash is about more than speed, it is about attention, reaction times, vehicular braking, keeping safe distances, and safe driving, not just ‘speed kills’. And therein lies one problem, that it is easier and less costly as well as profitable for government to get revenue from arbitrary rules than it is to patrol and take bad drivers off the roads and keep them off.
FWIW I always drive with my GPS running, and day in day out it shows a constant relationship to the digital speedometer display (within seconds of lags) depending on terrain and changes of speed. I trust it is probably a tad more accurate than the speedo at constant speed since when driving, if I use the GPS speed I am literally one of many cars travelling down the road together, but if I use the speedo I will soon be leading a parade, giving me cause to think my speedo might read a bit higher for any given speed than most. It could be everyone else on the road is technically speeding, but would it always be everyone but me?