17/01/2025
Is the speed limit tolerance of +/-10% km/h a myth?

Is the speed limit tolerance of +/-10% a myth or not?

OK, first what is a myth?

According to the Collins online dictionary a myth is "A myth is a well-known story which was made up in the past to explain natural events or to justify religious beliefs or social customs."

Based on that definition the +/-10% is not a myth, because at one time it apparently existed. According to this article about the Australian Design Rules, before 2006 the tolerance for speedometers was actually +/-10%. So if that's true, definitely not a myth.

I don't know about you, but for the life of me I can't see why cars just can't show the speed accurately. If I thought this was a conspiracy, this is a good way to get people thinking they're going one speed, when they're actually travelling slower. The slower people travel, the less chances of an accident, or less severe accident, and that meets the government's agenda of reducing the road toll.

However, there does appear to be relief in sight, as it appears the European Standard (which I believe is what we tend to follow) appears to be changing so that speedometers have to accurately show the actual speed the car is going. Of course this makes sense as self driving cars and technology advances. The more automated things become, the more accurate they need to be.

To me this is a good thing. Currently we have the situation where one group of drivers think their speedometer is showing the speed accurately, so they drive 4km/h or more under the limit, and another group of informed drivers, who if they wish, drive closer the the real speed limit. Whether that's by having their speedometer just above the speed limit, or using a GPS based device that shows a generally more accurate speed.

You may have noticed I said generally. GPS based devices are not as good as the speedometer in your car in a couple of situations. When a car is slowing down, or speeding up, there's about a second delay whilst the GPS device gets the information required. When the GPS signal is not available such as when in tunnels and we have a few speed cameras in tunnels. At times GPS devices just get spurious speed values. Whereas the speedometer in your car is consistentyly inaccurate. This reminds me of the old joke. "Which is more accurate, a clock that has stopped or a slow clock. The answer is a clock that has stopped. At least it's correct twice a day." Once you know how much your speedometer is out by, at a number of speeds, you can compensate for the differnce. For example my speedometer typically shows 4km/h above the actual speed between 60-100km/h. At slower speeds such as 40km/h, I feel the speedometer is out by more, but haven't yet quantified the amount. So your car's speedometer is still the most accurate device, as long as you mentally adjust the speed to take into account how it's been calibrated. If you'd like to see how far out your speedometer is, drive on a long flat road at a steady speed and use the Speed Limit Alerts web app avaiable on this site, to see your speed based on GPS readings, and compare that to your speedometer's reading. I'd do this for 40, 60 and 100km/h and note the difference you need to add to your speedometer to get the actual speed you're driving.

Now if you're one of those people who use your speedometer and drive at the speed limit of say 100km/h and people still pass you, sometimes they're speeding, but probably more often than not, they are driving at the actual speed limit based on a GPS device, or knowledge of their car's speedometer calibration. Those that rip past, well they're just speeding.

Kelvin

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