WheelHit.forwardSlip

public float forwardSlip ;

Description

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

using UnityEngine;

public class Example : MonoBehaviour
{
    // Prints "braking slip!" when tire slips badly.
    void FixedUpdate()
    {
        WheelHit hit = new WheelHit();
        WheelCollider wheel = GetComponent<WheelCollider>();
        if (wheel.GetGroundHit(out hit))
        {
            if (hit.forwardSlip > 0.5)
                print("braking slip!");
        }
    }
}