australian design rules speedometer accuracy

Under the current Australian Design Rules (ADR 18/03), a speedometer must never under-read, and may over-read by up to 10% + 4 km/h. In plain English: at a true 100 km/h, your speedo can legally show anywhere from 100 to about 114 km/h, but not 99. Many cars are set to read a little high to stay on the right side of the rule, and tyre size or wear can nudge the reading further. Older pre-2006 vehicles may have different allowances, so don’t assume they’re held to the same standard. GPS can help sanity-check, but it’s not flawless, especially in tunnels and canyons. Consult official documentation for exact figures.