Rigidbody_Velocity

Hello,I'm new to unity and I want to make a car game,I already have a fully working car from unity asset store.My problem is that I'm not sure about how to calculate it's velocity and then display it on my screen.The script that makes the car move is applied on the wheels.

 

Do you really want the car's velocity (as in, direction and speed at which the car body is moving)? Or do you want what a speedometer would show, i.e. the speed implied by the rotation of the wheels?

 

Is unity velocity a variable you can check and depend some other code on its value?
what is the type of it?
I mean ; could value of velocity be used to measure objects speed?
For example if i was using add force to move projectiles could i capture its velocity to measure how much damage it could inflict?