Question

does the speed of the vehicle depends upon the wheel size ?? if not why does all speedometers shows equal speed even if wheel size is different?

Answer 1

Speedometer on a vehicle is calibrated according to the wheel size of that particular vehicle.  Speedometers of different vehicles are calibrated or set differently.

Speedometer on a car is differently calibrated (set) according to the wheel size of that car. Speedometer on a motor cycle is set according to the size of the wheel of that motor cycle. If you interchange the speedometers from one vehicle to another, then it will show wrong values.

Speed of the vehicle does not depend on the wheel size.  Speed of the vehicle depends on the (power/force) thrust given by the engine, friction, wind speed and the load on the vehicle.

Calibration of speedometer

Speedometer increments a counter by 1 for each complete rotation of the wheel. When the counter value become = (1000 meters/ circumference of wheel), then the kilometer digit in the display increments by 1, and the counter is reset to 0.

Thus Calibration takes into account the wheel size.