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Coming from a background in mechical instrumentation, even a 5% tolerance is tight for a mass produced pointer instrument. The GPS is better than 1% error which is better than all but a reference mechanical instrument. Every part of the speedo system has a tolerance on it, The tyre tread, tyre pressure, wheel speed sensor, dash controller, pointer drive mechanism etc etc....
So +5% Full scale on a clock that shows 170mph is +8.5mph acceptable error at any speed. There is no need for them to be accurate, only to ensure you don't go faster than indicated. If it reads 60 when you're doing 50, whats the problem? except you'd pi## of other drivers. Your not going to break any laws or be able to sue anyone!!
It MUST OVER READ (show faster than actual speed)
Anyones speedo that UNDER READS (shows less than actual speed) is plainly dangerous and probably illegal for sure!!
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