How Accurate is Your Speedometer?

SpeedometerMany 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.

Reference Links:

Speedometer Scandal! - Car and Driver Magazine

Comments

Excellent article

This makes me wonder, is there a way then to test the accuracy of my speedometer?  Further, is there a way  to tune my speedo to match, say, a new set of tires that is different than my original set?  Aftermarket speedos?

Thanks for the info.

Adjusting Speedometer

Yes, the mechanics can adjust the computer that controls your speedometer to take into account changes in tires/wheels, a change in gearing or something similar.

How to check the accuracy of your speedometer

A good way to check the accuracy of your vehicle's speedometer is to use a GPS device that can display the speed. I recently installed wheels and tires on my pickup with a larger circumference and used a GPS to check the speed reading on the speedometer. I was pleased the difference was negligible. I had read on a forum dedicated to that particular make of truck the speedometers usually read 5-7 kilometres slower that the actual speed. Installing larger rims and tires corrected the builtin error.

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