Cant change variable speed in script

Hello, i am new in unity and i want to try to change speed from one script to another. Here first script. In this script i want to change speed and for this i using second script.

using System.Collections; using System.Collections.Generic; using UnityEngine;

[RequireComponent(typeof(Rigidbody))] public class MoveThinRoller : MonoBehaviour {

 public float distance = 1f;
 public bool instancedMaterial;

 private Rigidbody rb;
 private Collider col;

 private MeshRenderer mr;

 public float speed = 0.5f;

 // Use this for initialization
 void Start () {
     RefreshReferences ();

     ChangeSpeed (speed);
 }

 public void RefreshReferences(){
     rb = gameObject.GetComponent<Rigidbody> ();
     rb.isKinematic = true;
     rb.useGravity = false;
     col = gameObject.GetComponent<Collider> ();
     if (col == null) {
         col = gameObject.AddComponent<MeshCollider> ();
     }

     mr = gameObject.GetComponent<MeshRenderer> ();
     if (mr == null)
         mr = gameObject.GetComponentInChildren<MeshRenderer> ();
     if (mr == null)
         Debug.LogError ("Linear Conveyor needs to be attached to the belt Object");
 }

 // Update is called once per frame
 void FixedUpdate () {
     // 'Teleport' rigidbody back and Move forward with physics the same amount each frame
     Vector3 mov = transform.forward * Time.deltaTime * speed / distance;
     rb.position =  (rb.position - mov);
     rb.MovePosition (rb.position + mov);

 }

 public void ChangeSpeed (float _speed) {
     // change the speed of the physics and update the shader
     speed = _speed;
     // Create a new material instance
     if (instancedMaterial) {
         Material tempMat = new Material (mr.sharedMaterial);
         tempMat.SetFloat ("_Speed", speed);
         mr.material = tempMat;
     } else {
         mr.sharedMaterial.SetFloat ("_Speed", speed);
     }

 }

}

Here is second script, what i use for change speed in first script.

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class SpeedOff : MonoBehaviour {

 void Start()
 {
     
      
 }

   
 void Update()
 {
     
 }
  void OnCollisionEnter(Collision col)
 {
     if (col.gameObject.tag == "Box"){
       
         GameObject varGameObject = GameObject.FindGameObjectWithTag("ChangeSpeed");
        
         MoveThinRoller moveThinRoller = varGameObject.GetComponent<MoveThinRoller>();
         moveThinRoller.speed = 0f;
      

     
     
     
        
          
     }
 }

}

So its doesnt work. Can somebody help me? Thanks. p.s : Sorry for my bad english