Why does the speedometer fluctuate up and down?

[StLila]

Speedometer fluctuations are caused by a malfunction in the vehicle speed sensor. A faulty speed sensor can also lead to unstable engine idle, momentary stalling or engine shutdown when starting or decelerating to a stop, and reduced engine acceleration performance. The speedometer consists of two gauges: the speedometer and the odometer. The odometer is a digital instrument that calculates rotation through the engagement of components on the calculator's gear, primarily obtaining mileage signals from the speed sensor. The vehicle speed sensor is a device used to detect the speed of electronically controlled vehicles. The control computer uses this input signal to regulate engine idle, torque converter lock-up in automatic transmissions, automatic transmission shifting, engine cooling fan operation, cruise control, and other functions.