I'm sure it is closer to 100% ... else cars can't be registered. Manufacturers are only obliged to rectify inaccuracy if the displayed speed is less than actual, or greater by 10% + 4 km/h. So at an actual speed of 100km/h, the legal range to display (with genuine wheels and tyres) is 100 - 114 km/h. Not entirely. Whilst that is the rule NOW, between 1995 - 2006ish could have 10% either above or below the actual speed. It was only the manufacturers that wished to avoid lawsuits that did the above in relation to speedy accuracy. It's kind of interesting in a way that they've actually increased the allowable variance in speedometers. It also explains why so many people on the freeway these days drive so slowly