What is a speedometer?

The speedometer is installed on the instrument panel of a vehicle and measures the speed in miles or kilometers per hour. While the speedometers of some older vehicles are mechanically driven, eddy current devices and electronic speedometers are the most common. Neither technique uses tire rotation to indicate speed. Rather, the speed indications are scaled to the rotation of the transmission, which may be subject to some error, and, from time to time, calibration may be necessary.

Is my speedometer accurate?

If not calibrated to an acceptable degree of accuracy, a driver may exceed or go below posted speed limits. Excessive error can be caused by putting on after market modifications, including tires with nonstandard diameters, using different differential gearing, or other specification modifications.

What are my options?

The vehicle can either be taken to a local dealer or repair shop, or the owner can calibrate the speedometer. There are numerous automotive websites that offer DIY instructions, calibration testing and measurement tools for sale, and speedometer repair services.

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