They are not allowed to read fast..
If you were done for 33mph in a 30 zone your speedo would probably been showing 36.
if you don't believe this check your actual speed using a sat nav, these are far more accurate than a car's speedo.
As for the technicalities of the law, I don't know.
That said, speedometers are as prone to error as any device is. This is nowhere the issue it was decades ago when speedo's were mechanical. Today's electronics narrow that down pretty well and I'd be willing to bet that the speedo is close enough unless tire size, trans, differential ratio, etc have been altered from factory design.
Nine times out of ten, the greatest influence on speedometer reading is the right foot...
Legally a car's speedometer is not allowed to under-read (that is show a lower speed than you are actually travelling at), but can over-read by as much as 10%. So if you are travelling at 30mph your speedometer can legally show anything from 30mph to 33mph, but cannot legally show 29.9mph.
This makes them "failsafe" in that any error makes you slower rather than faster.
For this reason most manufacturers design their speedometers to over-read slightly (probably about 2%).
Note that if your tyres are worn and under-inflated the reduced rolling radius will increase the amount your speedometer over-reads by.
If you make any modification that would make your speedometer under-read (e.g. fitting larger-diameter tyres than standard) you would be breaking the law if you didn't have the speedometer recalibrated to take account of the difference.
If you attempt to use an inaccurate speedometer as a speeding defence be prepared to be prosecuted for driving an unroadworthy vehicle as well as for speeding.
having driven 18 wheelers I've learned a few things about the so called radars. 1st never admit to anything when you get pulled over. 2nd, use speedo's can be off and you don't rely on any factory settings as you don't know if someone put the wrong size tires on and cause it to be off. if you're not going with the flow of traffic you need to check it if it read faster that the avg traffic, do that more than once , the other thing is a lot of gps's now have a way to show your real speed and that is constant as it uses elect that has nothing to do with you car. so that is a way to verify your speed o. now there are several things a lot of people don's know, cops usually don't give tickets out unless you're doing 8 mph's or better according to their radar. that takes in account the for inaccuracy they might have as they do not verify the radars all the time like they say they do. cops got into law enforcement because of guns not radars. they know a lot about guns. they have to go to training for radars and even then they don' t understand dobler effect. in layman terms, a train whistle coming towards you gets louder, and when they go away for you they get weaker. thats the same as radar, it sends out a 1 gig signal and if nothing is coming towards or away from them, the same 1 gig signal is received. but if you're coming towards or a way from them, for every mph you're traveling, you have a think its 3 mhz difference than the 1 gig it sent, so it takes the difference and now converts it to mph's which they can see. that is way they have a stationary and a move mode switch on the units, as if they are moving then if uses their speed to figure yours out, but in stationary mode, it's not moving so it is what I told you before.
now there is one way to keep speeding off you license. you send a note to the prosecuting attorney saying if they make it a non moving violation, that you'll never speed in their county again and that you will plead no contre, or no contest, not admitting guilt, and you'll pay all court costs and fines. a lot of times it's less than the normal ticket. what happens it if they prosecute you for a speeding ticket, the state gets say 75% and the county gets 25% now if it's a non moving violation, the county gets all of the fine and they don't have to report to the state and have it on your record. win win for everyone. hope that helps.
they can be off about 2 or 5 miles an hour ! if you have a tackometer you can adjust your speed by it !
As an example, If I were to get a fixed penalty notice for 33mph in a 30mph zone and the police say they calibrate their equipment everyday and therefore 10% inaccuracy should not be accounted for, what in the case of the motorists speedometer? Or are speedometers completely fail safe?
