How Accurate is Your Speedometer?
Many drivers on the receiving end of a speeding ticket commented that their speedometer reported their vehicle's speed as something different than the radar or laser speed measuring device that I had used. I don't doubt that these drivers were telling me the truth. The trouble is that for virtually all of these incidents, I knew how accurate my speed measuring device was but these drivers had no idea whether their speedometer was accurate or not.
I asked Transport Canada what the standards were for speedometer accuracy. They regulate many things about how our vehicles are constructed and how their systems must function. The response was short and succinct: "Transport Canada does not regulate the accuracy of speedometers. If you are experiencing inaccuracies in relation to your vehicle speedometer, you should contact the original vehicle manufacturer."
Manufacturers are guided by a standard set by the Society of Automotive Engineers known as J1226 Electric Speedometer Specification. At speeds above about 90 km/h the allowable range for speed is 4% of the highest reading shown on the speedometer. For the vehicles in my family, this means +/- 8 km/h for my pickup and +/- 10 km/h for my wife's car.
Throw in some tire wear, improper tire inflation, a change of tires and wheels or even just a replacement tire of the same size classification and you can change this reading even more. If you like to travel at 10 over because the cops don't stop anyone for 10 over, you are easily risking being 20 or more over without knowing it. You may be well advised to stick to the number on your speedometer that matches the posted limit.