ADR's were changed a while ago now and the current formula is a tad hard to type out but is essentially...
The speedo cannot underread at all at any speed (IE you can't be going faster than indicated)
The speedo must overread by between 0 KPH + 4 KPH + up to 10% of the actual speed
EG at 80KPH actual the speedo must read between 80 and 92 KPH (80 + 4 + 10% of 80 which is another 8)
It is because of the 'new' ADR saying that it cannot overread that the Feds can get away with zero tolerance on speeding if they so choose