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Chris6791 said..
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Australian Design Rules state a speedo must read within 10% of actual speed BUT if it isn't 100% spot on it must err on the side of caution. So if it is out by the 10% and the speedo says 100 you may only be doing 90 or so. Speedo might read 60 but you might only be doing 55.
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That is partially correct, but fully correct in the point you are making. The point of this post is to clarify the universal whinge that speedos can be 10% out so it's not fair to be booked for doing less than 10% over.
There are actually quite a complex set of speedo accuracy rules depending on the type of vehicle. It's different for trucks and tractors and motorbikes and cars etc.
The actual rule for most cars is 10% + 4kph and as Chris points out the error has to be on the safe side. (You can google the ADR and read the formulae if you want.)
rvcs-prodweb.dot.gov.au/files/ADR%201803.pdfMore importantly, car manufacturers deliberately build the error into the speedo. The standard for Subaru is that the speed will read high by 5% + 2kph. That way you cannot possibly be booked if you are driving to the speed indicated by the speedo. More importantly for Subaru, you can't sue them because the speedo reads low and you had a speed related accident or got booked for speeding. My old car had a consistent 4% error so it was almost dead accurate at 60kph and read 104 when doing 100 kph.
The thing that annoys me is that they make the speedos perfectly accurate including the offset they build into it. In the old days it would have made sense to have a 10% allowance for speedos. With modern technology there's no reason to have any more than a 1-2% allowance to handle tyre pressure variations etc.
PS. For the nerds out there, think about the ramifications of building in a 5-10% error in the speedo. If it was simply a different calibration then your odometer would be out as well and that would affect your warranty and service intervals and all sorts of crap. The 5%+2 error would be proportional across the speedo so your speedo would be accurate at low speeds and less accurate at higher speeds. In fact on my car the offset is applied accurately at all speeds. I cross checked the odometer and the speedo against two GPS. The odometer is accurate. The speedo is accurate to the manufacturer's tolerance but not consistent with the distance and time measurement. There's a fair bit of processing going on between the sensors, the speedo and the odometer.