I think the tolerance limits comes out of an international standard, and in the UK the Police should take no action against speeders if the speed they record is less than the 10% +/- 2mph above the posted limit. I also believe that the fixed & mobile speed cameras work the same way. As the UK is going to see a massive expansion of 'average speed' cameras from 2009, which log your registration number and if it appears again on another camera within up to 30 miles calculate the average speed over that distance it is going to be very important. The UK is becoming a real "police" state in regard to this area - personally I would rather see police on the streets than a camera any day.

I also belive that there is a correction system available if the error is 'out of limit'. I had a VW where the electronic speedo was replaced and the new unit was way out (over-reading by 15mph @55mph) according to my GPS. The dealer plugged in a computer and entered a correction factor and from then on the speedo was always within 4mph of the GPS even up to 100mph.

One question I have always wondered about though, is whether the odometer is driven from the speedo (as they always used to be when the were cable driven) in which case a speedo over-read of 10% actually means that your odo also over reads by the same and you have not actually done all those miles. Anyone know the answer as to how this all works on modern all electronic systems above the posted limit

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