how to get car speed ???

I have car with move controller and i want to get the speed of it…

i use this method :

carSpeed = (2*Mathf.PI*(FrontLeftWheel.radius*100)*FrontLeftWheel.rpm/100)*60/1000;

also i use this :

carSpeed = (2*Mathf.PI*FrontLeftWheel.radius*FrontLeftWheel.rpm/100)*60/1000;

carSpeed = ???km/hour

But i think that wrong,because car Speed retch very high number or very low.

car Wheel radius is = 0.45

Multiplied by 100 to convert from meter to cm

For a car, speed in km/hr is