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.
10 Over is (NOT) OK
Drivers have learned that traffic police don't ticket them for speeding until they are well over the limit. The original photo radar program promised that tickets would not be issued for less than 10 km/h over the limit. This means that driving 10 km/h faster than the limit is OK, isn't it?
It's not OK, especially if your speedometer is not accurate. Have you ever tested it for accuracy?
Transport Canada regulates many things about how our vehicles are constructed and how their systems must function in the Motor Vehicle Safety Regulations:
Speedometers and Odometers
(4) A speedometer shall indicate the speed of the vehicle in kilometres per hour or in kilometres per hour and miles per hour. The unit or units of measurement shall be identified on the speedometer or at a location adjacent to it.
(5) A speedometer shall be illuminated whenever the headlamps are activated, unless the headlamps are being flashed for signalling purposes or are being operated as daytime running lamps.
(6) An odometer or trip odometer shall indicate distances in kilometres or in miles. If the distances are indicated in miles, that unit of measurement shall be identified at a location adjacent to the odometer or trip odometer.
I asked Transport Canada what the standards were for speedometer accuracy. 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."
Society of Automotive Engineers
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.
Modifications, Wear & Tear
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. Remember that 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.