[Unity 5.6] Problem with WheelCollider

Hi guys, i have a problem with wheelColliders. I think is a conflict with a box collider. In the hierarchy i create an empty GameObject, my child are the body of car, the physic wheel and the wheelColliders.
For calculate the current speed i follow a tutorial and the formula it's:

Code (CSharp):

1. engine.currentSpeed = 2 * (22 / 7) * wheelsCollider[2].radius * wheelsCollider[2].rpm * 60 / 1000;
2.         engine.currentSpeed = Mathf.Abs(Mathf.Round(engine.currentSpeed));

The wheelCollider[2] it's the BackRightWheel. When i play the current speed isn't linear but seem a random value. If I remove the Mathf.Abs the value go to negative number. How can I solve this problem.

My car configuration:

- MaxTorque: 50;
- MaxSteer: 20;
- TopSpeed: 300;
- DecellerationSpeed: 30;

The currentSpeed stay in a value between 0 - 100.

Thank's in advance.