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# What is Speedometer Calibration?

Article Details
• Written By: Carol Francois
• Edited By: Bronwyn Harris
• Last Modified Date: 14 April 2018
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Speedometer calibration is the mechanical process used to validate speedometer output against the true vehicle traveling speed. A speedometer is a tool of measurement commonly built into the dashboard of every motorized vehicle or boat. The purpose of a speedometer is to provide an accurate indication of traveling speed to the driver. There are two types of speedometers: analog and digital.

When any motorized vehicle is assembled, a speedometer calibration is done. The factory staff sets the speedometer gear to correspond with the ring and pinion ratio as well as the tire size of the vehicle. The speedometer value is set using a hairpin or a strong magnet. Speedometer calibration takes into account the gear ratio in the drive cable, the size of the tires and the final drive ratio in the differential. All three items have an impact on the accuracy of the speedometer.

The gear ratio determines the rate at which the speed can increase or decrease and is independent of the transmission type. The tire size is the most important factor in the correct calibration of a speedometer. One complete turn of the axle results in one complete revolution of the tire. If you change the tire size, the vehicle will cover more or less ground in the same tire revolution.

It is important to remember that speed is the value of distance traveled divided by time. The speedometer provides information based on the distance covered in one tire rotation and the amount of time required to complete the rotation. A change in the dimensions of the tire alters the amount of ground covered. The speed reading provided by the speedometer will be lower than the actual speed the vehicle is traveling.

The final drive ratio in the differential determines how the vehicle to responds to acceleration or braking requests. The rate of change has an impact on the speedometer calculation of speed and may result in incorrect information if the ratio is incorrect. When the tire size of a vehicle is changed, a speedometer calibration must be completed. This correction can be done by adjusting the hairspring or modifying the strength of the magnetic field. To modify the magnetic field, use a powerful electromagnet to adjust the internal settings of the speedometer by moving the magnet towards and away from the speedometer.

Check the results carefully to ensure that they are correct. Test the results by comparing the speedometer value to the input from the rotating drive cable. The special equipment required to access data from the rotating drive cable is available at your local auto mechanics shop. This simple test is very quick and provides assurances that the speedometer is correct.