Question about motorTorque

Quick question about motorTorque:

When motorTorque is powering wheel colliders to move a rigidbody, what is Unity using to measure that object's speed? Is it Rigidbody.velocity or is motorTorque using something else to move an object? I'm wondering how to monitor whatever variable this is, so as to use it for scripting engine sound pitch and to control acceleration and max speed.