Speedometer Error from Tyre Size Calculator NZ

This calculator shows how changing your tyre or wheel diameter affects your speedometer in New Zealand, giving your true speed and the percentage error at a given indicated speed. A speedometer measures how fast the wheels turn and converts that to a speed using the original tyre diameter, so the moment you fit tyres with a different overall diameter, the reading is off. Larger-diameter tyres cover more ground per revolution, meaning you are actually travelling faster than the speedo shows, while smaller tyres make the opposite true and the speedo over-reads. The error is simply the percentage difference between the new and original diameters, and it carries straight through to your odometer and trip-distance readings too. You enter your original tyre diameter, the new diameter, and an indicated speed, and the calculator returns your actual speed at that reading and the error as a percentage. Use it when fitting plus-sized wheels, larger off-road tyres, or a different profile, to understand whether your speedo will read high or low and by how much, since an unnoticed error can lead to unintended speeding. A change of a percent or two is minor, but larger changes are noticeable and may affect your warrant of fitness if the tyres differ too much from specification. An estimate based on diameters.

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A positive error means the speedo under-reads and you are going faster than shown. Large changes may affect your warrant of fitness. An estimate from diameters.