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!");
}
}