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.
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.
The actual speed at a given indicated speed is the indicated speed times the new diameter divided by the original diameter. The speedometer error is the percentage difference between the new and original diameters. A larger new diameter gives a positive error, meaning the speedo under-reads and your true speed is higher than shown.
Going from a 632 millimetre to a 645 millimetre diameter is a 2.06 percent increase. At an indicated 100 km/h you are actually doing about 102.1 km/h, so the speedo reads low by about 2 percent.
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