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THE Australian Design Rule calls for a speedo to be accurate to plus or minus 10 per cent, so at 100km/h your speedo could be showing anything from 90km/h to 110km/h. If it is showing 112km/h when you're actually doing 100km/h, it doesn't comply with the standard. The problem I have with your dyno test is that you don't know the accuracy of the dyno. It is also a machine with tolerances and you need to know what they are to determine the results' accuracy. Remember the speedo is built with tolerances so is unlikely ever to be 100 per cent accurate. Many other factors affect the accuracy of the speedo reading: the rolling diameter of your tyres, wear on tyres, inflation pressure and so on. The RACV regularly checks speedos for accuracy and results show speedos of modern cars are much more accurate than those on older cars. Most speedos checked fall on the conservative side -- that is, the indicated speed is higher than the actual speed.
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