I'm not referring to the 10% tolerance on the speed camera, but the speedo in the car.
Okay most speedo's under read the actual speed, but a cars speedo is not a regularly calibrated instrument so it's unfair to assume that it's reading the correct speed, hence my understanding of a 10% tolerance! an extreme case of the speedo indicating say 40mph, but you are traveling at 44mph.
Yes, sure; but depends what we mean by "10% tolerance". That would be true if the accuracy is "within 10%" i.e. + or -10% (which is a huge variance for any instrument worth the name) or "a 10% zone" i.e. + or - 5%. But neither is the case: as I understand it from elsewhere, and the quote seems to say, the tolerance is in the drivers favour - speedo always overestimates true speed, so it's not around 10% each way, it's just around 10% on the downside so you will always be doing less than the indicated speed, not more. Not surprisingly that tolerance is I believe deliberate (i.e. not machine inaccuracy, but added in or built in as the quote says) this way for reasons of legal liability: manufacturer not being sued by (probably American!) driver because driver claims "your speedo told me I was doing 70 but I got nicked for doing a measured 77 and lost my licence, hence lost my job, hence you owe me a zillion dollars compensation"...etc
But none of this tolerance is actually specified legally I think so you can't rely on it to take to court; and I guess we can't assume Maserati do it.
Of course if you fit non-standard wheels and/or tyres and change the rolling radius of your wheels you are in a whole new area
With the rise of GPS and built in connectivity, I wonder if this will remain the case or if we will have future speedos that don't measure anything on the car at all, just track satellite signals? Probably not for a while if ever, manufacturing regulations tend not to like things that are outside their control.