I'm not sure what standard this refers to, but I dug up this article I remember reading previously (it doesn't appear to be published on the site anymore)

The applicable standard for many vehicles sold in NZ is a European standard that specifies that speedometers must not indicate a speed less than the vehicle’s true speed, or a speed greater than the vehicle’s true speed by an amount of more than 10 percent plus 4 km/h.

If true, that means a speedo is considered legally accurate if it says 114 when you're driving 100, which is a pretty massive difference.

Bigger tyres is a nice workaround, both our cars over read by 4kph so I just do the maths.