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Now that there is a zero tolerance to driving over the limit, I need to know if my speedo is absolutely accurate, reading high, reading low, the same at all speeds, the same in hot weather and cold weather, when I first start driving and when my tires are warmed up. To comply with the new rules, I believe I need to know these facts and compensate accordingly for every condition for my car and the other cars I drive. It seems impossible to me as speedo's are not that accurate. So do we all need to drive at speedo reading 45 in 50 K zones to ensure no ticketing? Please, we all need a definitive answer how to ensure compliance to this new ruling. Thanks. PS, we all need this information yesterday as they began this new strategy with no warning to prepare for it.

Hi there,
It is illegal for a vehicle speed to show they are travelling slower than "true" speed, so manufacturers build into a vehicle the opposite.
The motor industry standard deviation is 10%+4kmh.
So if you had a GPS determined "true" speed reading of 100kmh- a vehicle speed display can be anywhere from 100kmh to 114kmh.
There can also be a smaller deviation at 50kmh than at 100kmh on some vehicles.
Some people use a GPS to determine what their vehicle displays at the different speed limits and adjust their driving accordingly, or if you follow what the vehicle is displaying and for example drive at 50 on the display in a 50 zone then there is a high chance you won't be speeding.

Thanks. That is very useful information. Too bad this is not advertised when you buy a car! Anyway, I have downloaded a GPS app for my phone. I hope is accurate, so I can work out what speed I am really driving at. I'm not a fan of such grandiose deception. Cheers and thanks.