Your speedo probably isn’t accurate, here’s why

We’ve all been there, cruising down the highway, speedo sitting on 100kh and next minute a car speeds by, seemingly leaving you in the dust.

Then another, then another, then another.

Everyone else on the road, especially those speed limited trucks must all be speeding, double trouble for those truckies that have huge fines for speed tampering on their vehicles.

But hey, you fit some bigger mud tyres and the issue goes away.

Wait….does this mean now you are speeding? Well, go out and buy a GPS HUD or crack open a GPS app on your phone and you’re now….doing the speed limit? How does this work?

If your car was built since 1989 you need to look towards Australian Design Rule 18 – Instrumentation, which it’s own explanatory statement says:

The function of this Australian Design Rule is to specify requirements for the provision of speedometers.

And if you download the entire ADR and have a read through, you will find the following little tidbit of information, which is actually great for us as 4WD owners, especially when we go fitting bigger tyres: indicate the actual vehicle speed, for all speeds above 40 km/h, to an accuracy of ± 10 percent

ADR18/00 Para

So this means that for any speed that you are doing, if you are doing above 40kmh, your speedo is legally allowed to be out by 10% in either direction (Hence the use of the ± sign.

So what can my actual speed be?

Have a look at this chart to see what your speed range is allowed to be as per the ADR:

Posted/Displayed Speed Limit (km/h)Minimum Allowable Speed (km/h)Maximum Allowable Speed (km/h)

I mean, at 50kmh doing 55kmh isn’t such a big deal, and neither is doing 45kmh, it’s really stuff all when all things are considered. However if you are on the Hume Freeway, or the Bruce Freeway where the speed limit is 110kmh, with people doing between 99kmh and 121kmh that’s a speed difference of 22kmh which is a decent crash speed if you’re the slow car in that instance.

What is even more concerning is that in some states like QLD having dropped the 10% tolerance for the issue of speeding tickets now means that you are having to take a completely legal vehicle to testing stations and get them spun up and a calibration certificate produced so that you can contest a ticket in court.

But is my car illegal if I’m reading -9%?


Now that a lot of state laws are starting to provide lower and lower tolerances on speeding allowances, and a lot of the legislators saying that the onus is on you to ensure that you are not speeding, it is essentially being a cop out from the government that the onus is on you to ensure that you are not speeding, and they don’t care what your speedo is actually reading, and that the accuracy of the speedo is irrelevant.

So what can I do?

Pay someone to calibrate your speedo

Well there are a number of things you can do, the most simple being is to go and get your speedometer calibrated correctly, so a quick google search can really help you find your local businesses that can calibrate your speedo.

Calibrate the speedo yourself

MightyCarMods did a great video on this a few years back, and we are amazed that many people haven’t seen it, this solution will work on a lot of modern vehicles with electronic speedometers

Buy a Speedo Correction Gear!

With some simple math, you can easily work out how much your speedo is out, the one that we normally use is to get our GPS out and see what the speed is at 100kmh, so if you’re doing 97kmh when your speedo reads 100kmh, that’s 3%

You can go to companies such as Mark’s 4WD and get Speedometer Adaptor Kits to help convert speeds and make your speedo accurate.

Just use your phone GPS

Provided you have a half decent phone that uses actual GPS or “High Accuracy” Location, you already have in your pocket a fairly good speedo, there are many many apps out there that will tell you your speed. Some of our favorites are:

However your phone can’t just sit by itself in your lap, with sitting your phone in your lap or in the centre console now being disallowed by a lot of states, you need a “Fit for purpose” or “Designed for the function” mobile phone holder

To that end, here at 4WD DIY we stock the following awesome phone cradles from RAM Mounts

Now, if you want to fit something different, such as any number of off the shelf dedicated GPS units, we also have cradles for them:

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