Is one still allowed 10% speedometer error?
I doubt it. Anyway, I understand that the "10%" error was meant that speedometers could OVER read by that much when installed, but still be considered functional. For example, if it shows 33 mph then the actual speed is 30 mph. This was because speedometers were mechanical devices, with gear trains that were not that precise. They should never under read, which could lead to law breaking and fines.
For many decades I have checked the accuracy of the speedometers of the vehicles I have driven - from cars and vans up to 7.5 tonne lorries - against the roadside markers on motorways. They have always over read. My purpose was to drive at the maximum legal speed, especially when driving for a living and having deadlines to meet. Average speed cameras mean that many drivers drive below what their speedometers indicate, and can thus be doing 45 mph in a 50 zone - highly irritating
if they are in an outer lane.
Nowadays, one can use a satnav to check the actual speed of a vehicle. For example, my VW Fox's speedometer reads 33 mph when the car is doing 30 mph, 75 mph when doing 70 mph. I have put small markers on the speedometer at 33 mph and 44 mph, knowing if the needle is on these I am still not speeding.
There once was a general rule that speed cameras would not take record a vehicle if it was no faster than 10% plus 2 mph above the limit. For example, 35 mph in a 30 zone. But many local authorities have cut this down, even - it is said - to anything over the limit, even 31 mph in a 30 zone. Not so much for safety reasons, but for cash income from speed cameras.
I once had a young friend who had just passed her test on a scooter, and was so
worried about going over the speed limit that she was always taking her eyes off where she was going to check her speed. One day she did just that - and had an accident. Why head-up speed displays are not normal on any vehicle with a windscreen, I do not know.
Worth knowing is that a speed camera should only function when a speeding vehicle is travelling in the direction that the yellow-painted side of the camera faces the driver. If the grey side faces the driver then it should not take a record of a speeding vehicle. Of course, some cameras have both sides painted yellow.