Why a margin for error factor? Because the variation in the tire pressure from hot summer to winter cold snap (I've seen 100 degree difference in 1 day) can change the speedometer reading to actual speed over ground by 10%.
A cop won't bother pulling over anyone for just 2 kph, or mph, because they are instructed to stop people speeding in the higher fine brackets. Like 10 to 15 mph over, when the fine doubles.
https://www.facebook.com/MeanwhileInAustralia/photos/a.388527251219887.92144.388525541220058/1004060196333253/?type=3
