WheelHit.forwardSlip

public float forwardSlip ;

Description

Tire slip in the rolling direction. Acceleration slip is negative, braking slip is positive.

    // Prints "braking slip!" when tire slips badly.
    function FixedUpdate() {
        var hit : WheelHit;
        var wheel : WheelCollider = GetComponent.<WheelCollider>();
        if( wheel.GetGroundHit( hit ) ) {
            if( hit.forwardSlip > 0.5 )
                print ("braking slip!");
        }
    }