Speedometer calibration ensures accuracy of the speedometer by adjusting it to correspond with the gear ratio, tire size, and final drive ratio. Changing tire size affects the accuracy and requires recalibration. Testing the results is important to ensure accuracy.
Speedometer calibration is the mechanical process used to validate the speedometer output against the true ground speed of the vehicle. A speedometer is a measurement tool commonly built into the dashboard of every motor vehicle or boat. The purpose of a speedometer is to provide an accurate indication of the speed of travel to the driver. There are two types of speedometers: analog and digital.
When any motor vehicle is assembled, a speedometer calibration is performed. Factory personnel set the speedometer gear to correspond to the ring-and-pinion ratio as well as the vehicle’s tire size. The speedometer value is set with a fork or a strong magnet. The speedometer calibration takes into account the gear ratio in the driveline, the size of the tires, and the final drive ratio in the differential. All three elements have an impact on the accuracy of the speedometer.
The gear ratio determines the speed at which the speed can increase or decrease and is independent of the type of transmission. Tire size is the most important factor in correctly calibrating a speedometer. One full turn of the axle results in one full 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 the speed is the value of the distance traveled divided by the time. The speedometer provides information based on the distance traveled in one tire rotation and the amount of time required to complete the rotation. A change in tire dimensions alters the amount of ground covered. The speed reading provided by the speedometer will be less than the actual speed the vehicle is traveling.
The final drive ratio in the differential determines how the vehicle responds to requests for acceleration or braking. The rate of change has an impact on the speedometer speed calculation and can lead to incorrect information if the ratio is incorrect. When changing the tire size of a vehicle, a speedometer calibration must be completed. This correction can be done by adjusting the hairspring or by changing 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 toward and away from the speedometer.
Check the results carefully to make sure they are correct. Test the results by comparing the speedometer value to the rotary drive cable input. The special equipment required to access rotary drive cable data is available from your local auto mechanic shop. This simple test is very quick and ensures that the speedometer is correct.
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