Drag acting over rigidbody

Does somebody know how the drag parameter acts over a rigidbody? Is it velocity-dependent? I'm preparing a car simulation, but using the rigidbody's drag outside of the other simulation functions is being quite troublesome. Thanks in advance.

From what I’ve observed the effect of the drag parameter seems to be the same as rigidbody.AddForce(rigidbody.velocity * dragParameter * -1); if it gets called in FixedUpdate.

From what I’ve observed the effect of the drag parameter is closer (but not identical) to rigidbody.AddForce(rigidbody.velocity * rigidbody.mass * dragParameter * -1); if it is called in FixedUpdate. (Which doesn’t make much sense to me.)