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.

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

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.

Speedometer Accuracy

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.

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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.

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.

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.

I think manufacturers will tend to err on the side of caution (liability?) making sure their speedometers are optimistic (indicating you are going faster than you really are, so you wont try suing them if you get a ticket). While i have had at least two speedometers that were spot on, most have been optimistic (the majority of my speedometers have been, and two still are, mechanical rather than electronic)

I can't believe that in this modern age that car manufacturers are allowed any error out the factory door. Years ago I had a friend with a brand new Nissan Altima that read 13k slow right from the factory

And of course the officer call Bullshit! Why on earth would the owner of a brand new car have to worry about his speedometer reading?

The inaccurate part really I think just comes from the interpretation of the SAE regulations. While the SAE's regulations are not law, they're often referred to when new legislation is drafted down in the United States, or when legislation is reviewed over time. So for example, while the SAE may seem to read that the speedometer must be accurate to within 4% of the highest number displayed on the speedometer, when we take a look at the US FMVSS it gives us some bonus context for what the SAE was likely referring to.

The US FMVSS CFR 393.82 regarding speedometers reads: "Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to with-in plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)." You can check this out in the US standard.

Considering the history of consultation that the SAE's own regulations and policies have had on shaping US laws it's very likely that the SAE is referring to an accuracy of 4% of the indicated speed, rather than the top speed of the speedometer. I'll give you a very extreme example that would suggest the interpretation from the US FMVSS is likely the intended messaging from the SAE as well.

A 2019 Corvette ZR1 in Canada has a top speed of 360KM/H displayed on the speedometer. By the interpretation stated in the article this would mean that a if this vehicle was moving down the road and displaying 90KM/H as your speed of travel the vehicle in question could be moving as much as 14.4KM/H(360 *0.04) faster or slower than the 90KMH. At 104.4KM/H in a 90KM/H zone, that's a very extreme amount to be out, to a point where the local mounties may want to have a word with you regarding your speed of travel. Now if one was to interpret the SAE's guidelines to suggest that it's a margin of inaccuracy of 4% of the speed of travel displayed by the speedometer, that would mean when your speedometer displayed 90KM/H for a speed of travel you'd be moving +/- 3.6KM/H of the speed displayed by your speedometer. 93.6KM/H I wouldn't think is enough to make the mounties turn their heads too much and in all likelyhood they'd assume another issue such as tire wear, inaccuracy of your speedometer, or oversized tires are more likely to be the culprit rather than malicious intention to speed.

Now of course the information I've stated above hinges on an interpretation of the SAE's policy rather than hard evidence. So let's get down to the exciting part! As an instrument cluster repair and service facility we actually produce emulators that allow for us to essentially "drive" an instrument cluster on an isolated work bench environment. Through these emulators it allows us to test the accuracy of speedometers, tachometers, fuel gauges, temperature gauges and much more. We can find that almost all instrument clusters produced with Canadian market speedometers follow the FMVSS's guidelines for accuracy. Even when we take a look at European automakers who would not be primarily developing an application for the US market and have Canada as an after thought such as Volkswagen or Audi, we can generally see they will also follow the FMVSS's guidelines for accuracy regardless of if the top speed displayed on the speedometer is 200KM/H or 500KM/H.

Mitchell Realff

General Manager

Kelowna Instruments