Check Your Speed


In Australia, before the year 2006, speedometers could legally be plus or minus 10%. This means that a speed of 100km/h the speedo could indicate anywhere between 90km/h and 110km/h.

Some of those 2006 cars are still on the road. Many of them have had tyres replaced, which can put the speedo out even further. Most states with roadworthy inspections do not test the accuracy of the speedo at all.

After 2006, the law was changed to read – The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h. However this law applies when vehicles *arrive* in the country – once they are sold, nobody checks any of this again as far as I am aware.

So when I got my car in 2010, I tested the speedo with satellite apps and GPS. My car was officially 10% out. If I was doing 100 as seen on the speedo, I was really doing 91. And I adjusted my driving accordingly so that I was not constantly being overtaken by everyone.

After 5 years, we noticed my tyres had begun to craze. So we replaced them. Not long after that I tested my speedo again and it was now correct instead of 10% out. So I had to unlearn all my adjustments.

How many people reading this have done any checks on their speedometer? Does anyone reading this know for certain whether their speedo is accurate? Do you test it regularly? Did you know about these laws especially the one before 2006 if you have a 2006 vehicle? Did you know that you could be driving 10% slower than you expected in a brand new car you just bought and drove off the dealers yard, and that by changing the tyres you could suddenly be doing a new speed as a surprise to yourself, even though the speedo shows you the same speed?

I am guessing the answer to all of those questions is no. And this is how a lot of people get fined and accidents get caused because people are going slower than they think they are, and the person behind gets frustrated and road ragey.

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6 thoughts on “Check Your Speed

  1. I didn’t know this about speedometers and I’m sure there are many, many people out there who also don’t know. I don’t drive so I trust the bus drivers to know what they are doing, but I hope motorists read this and pass the word on.

    • I would suspect it is less than 20% of road users who are aware of this speedo thing, River. Some people might have found out when they got a GPS that shows them their speed, but most people are unaware. :( I only found out because The Other Half had an app which told all kinds of car info as you were driving and he plugged it into my car one time, I was shocked to discover it but all of a sudden those people overtaking me while shaking their heads all made sense. :(

  2. Even our newest car, a 2015 model, reads well over, 94 when it displays 100, 50 at 53. I usually sit in between the difference. I did know about the law but I didn’t know that tyres made such a huge difference.

    • For some reason, Andrew, the people where I live get quite road ragey when they get stuck behind someone not doing the exact posted speed limit. I have seen some deeply terrifying driving as a result. Partly this could be we do have some very long stretches where there is no place to overtake. I find it especially bad when it is a learner driver who the government has seen fit to handicap at 80km/hr even on a 100km/hr road, especially when only the car right behind knows it is a learner due to the local habit of sticking L plates underneath the numberplate. I really disagree with that law – either they are capable of being on the road at all the speeds we can do on the road, or they are not. :/

    • Mitchell – I would say at least 80% of people have no idea their speedometer can be out by this much. It is a real problem on our single lane 100km/hr roads here, where folks think they are travelling at the speed limit and are in fact doing somewhere around 91km which is nearly 10km less than the speed limit. If you overtake them, they get all indignant when really they should not! ;)

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