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Speedometers are legally required to never under-read, so they typically over-read by 2-5%: an indicated 78 km/h may be 76 actual. Enforcement tolerances are separate and discretionary, commonly 10% plus 2 km/h, and are never a legal entitlement.
Speed check
actual = indicated / (1 + speedometer error); threshold = limit x (1 + tolerance) + 2
Regulation requires that they never show less than actual speed. Manufacturers build in a margin so tyre wear and pressure variation cannot push them under.
No. It is guidance to enforcement agencies, not a legal allowance. The limit is the limit.