The legal speed limit that is aloud is actually 10% over the speed limit so motorway travelling is 77. So someone travel in a non mb car at there speedo reading of 70 is actually travelling at 64ish and someone travel with sat nav speed (gps) 100% accurate of 77 is actually travelling 13mph faster which is a hell of a lot of difference. Imo 75% maybe more of people that overtake you on the motorway have gps speedo.
I say non mb cars because every mb I have driven with sat nav speedo reading has been almost perfect, oh apart from my police vito that was calibrated at new and that is 2 mph slower at 70
A speed limit of 70 mph is exactly that - at 71 mph you are breaking the law ; you are most certainly not 'allowed' to go any faster .
Please note I said nothing in any post about indicated speeds - it is up to you to determine your actual speed , although the instruments in many modern cars are much more accurate than those of days gone by .
Electronic speedometers which work on the pulses generated by ABS sensors do tend to be more accurate than mechanically driven ones .
GPS , in theory , ought to be more accurate , but is not always so due to delays in extrapolating time and distance data to calculate speed , errors caused by driving up or down inclines or around bends - so they are not as infallible as some may think .
Different areas each have their own prosecution thresholds , which is quite different from saying that anything over the posted limit is legal : it is not - the offence of speeding is absolute and technically you are breaking the law at one mph over , even though prosecution is unlikely at that level .
ACPO guidelines suggest a threshold of 10% + 2mph over the posted limit before prosecution ; different areas then have different guidelines for conditional offer of fixed penalties , court appearance or more serious charges than speeding being considered . Here in Scotland , the local Procurator Fiscal will decide on charges appropriate to the circumstances . A former member of this forum recently reported a case of 138mph on the M74 resulting in a dangerous driving charge .
In England & Wales there can be an offer of taking a speed awareness course up to a certain level , then fixed penalty notice , with a summons being mandatory over a certain margin - these margins vary from one force area to another as a matter of local policy .