// Change the material depending on the speed of rotation
var fastWheelMaterial : Material;
var slowWheelMaterial : Material;
function Update () {
    if (rigidbody.angularVelocity.magnitude < 5) {
        renderer.sharedMaterial = slowWheelMaterial;
    }    
    else {
        renderer.sharedMaterial = fastWheelMaterial;    
    }
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Material fastWheelMaterial;
    public Material slowWheelMaterial;
    void Update() {
        if (rigidbody.angularVelocity.magnitude < 5)
            renderer.sharedMaterial = slowWheelMaterial;
        else
            renderer.sharedMaterial = fastWheelMaterial;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public fastWheelMaterial as Material

    public slowWheelMaterial as Material

    def Update():
        if rigidbody.angularVelocity.magnitude < 5:
            renderer.sharedMaterial = slowWheelMaterial
        else:
            renderer.sharedMaterial = fastWheelMaterial