Adr's specify there can be up to a +10% error margin, so the speed can read 10% above your speed, but not below. It is pretty slack even for the adr's to have such an error allowance, but car manufacturers get it 10% out and just say oh well, that's what the rues state it can be. It is pretty sad given the technology we have now that they can't get it to read closer. At least you won't get done speeding if you sit on the indicated speed, lol.
The only issue with putting bigger tyres on is the odometer reading will be out, it will say you have travelled less km than you actually have.