The speed is calculated by a wheel speed sensor. Depending on how fast your wheel is rotating your 'speed' can be all over the place. If you get oversized wheels and tires it will read low. If you get ones that are too small, it will read high.
It all has to do with the diameter of your wheel. Bigger wheels rotate less in a given distace, smaller wheels more. All you have to do to change how your speedometer reads is get bigger/smaller tires and wheels
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