If the Speedo is under reading then they have changed the tyre profile to something larger. New ADR 18 says speedos cannot indicate less that the true speed, and over-indication accuracy is limited to a maximum 10 per cent plus 4km/h. A vehicle's speedometer must be kept free from any obstruction that may prevent it from being read and shall at all times it is used on a road be maintained in good working order. The current UK law is based on the EU standard with some minor changes… A speedometer must never read less than the actual speed or show more than 110% of actual speed + 6.25mph. But if Officer Aircop starts the stopwatch a second too late or stops it a second too early and gets 5.9 seconds, he incorrectly figures Dawn's speed to be (0.125 mile/5.9 sec.) A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. "The accuracy of the speedometer in most vehicles, including Volkswagens, is generally within a few percentage points of actual speed," Tetzlaff … The implications are that it must never under-read - for obvious reasons - but may over-read. That is, at a true vehicle speed of 100km/h the speedo could indicate between 90km/h and 110km/h. ACCURACY LIMIT. If a speedometer is not accurate, it could display a lower speed than the vehicle is actually travelling at, thus causing drivers to believe they are not speeding when in fact they are. A small snippet from The Car Expert about car speedometer accuracy (UK specific): The UK law is based on the EU standard, with some minor changes. Speedometer Accuracy. Speedometer Inaccuracy, Radar tolerances and the Law . So, if your true speed is 40mph, your speedometer can legally read up to 50.25mph, but never less than 40mph. This means that at a true speed of 100 kilometres per hour the speedo can’t be displaying 99, … Vehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of -0->+10%. ‘Speedometer’ means that part of the speedometer equipment which indicates to the driver the speed of his vehicle at any given moment (2). An odometer accuracy of +/- 4 percent was also a requirement. EXAMPLE: Officer Aircop sees Dawn Driver pass between two markings an eighth of a mile apart. Speedometers and the law. 2.5.1. Construction and use regulations specify your speedo can over-read by 10 percent, but under-read by zero percent The threshold is there, effectively, so that drivers have no excuse if caught. ACPO Guidelines on speeding, and the UK law regarding prosecution. In the speedometer's case, here's my guess of how it probably works. Something like a change from 195/65/R15 tyres to 195/75/R15 tyres is what seems to have happened. 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