What is the tolerance allowed for the innacuracy of a speed limiter?
i.e If it is set to 90km/h, what is the tolerance for inaccuracy and how is this tested and how is it enforced ?
If anyone can point me towards any legislation on this, i’d be grateful.
The speed limiter won’t be the problem. The problem would be the speedometer. Taken from here.
As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles’ maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.
Why, you had a ticket 
No ticket. I’m just wondering how two trucks (same make with the same tyres) set at the same speed on the limiter actually travel at different speeds i.e one slightly faster than the other pulling the exact same empty trailer. I’ve also seen a sticker in the cab detailing the speed limiter set speed with an indication of +/- 2km/h so was trying to establish if there was a tolerated margin of error set in regulation.