AddForce with speed variations doesn't works properly?

I have a C# script where I set using AddForce () the moving speed for my player, and a script that makes it possible to upgrade the speed during game. The issue is: when I make a reference from a script to another, and Play my game, my player does not move as fast as he should! This is the script:

 private Rigidbody rbody;
 private CanWalk canWalk;

 [SerializeField]private float speed = 5000f;
 [SerializeField]private float angularSpeed;

 [SerializeField]private float speedMultiplier;
 [SerializeField]private float speedDivisor;

 [SerializeField]private KeyCode runButton;
 [SerializeField]private KeyCode crowtchButton;

 private float inputV;
 public bool invertVertical;

 private float moveZ;

 private bool cheat;

 private bool speedUpgrade;
 public bool SpeedUp {get {return speedUpgrade;} set {speedUpgrade = value;}}

 void FixedUpdate () 
 {
     rbody = GetComponent<Rigidbody>();
     canWalk = GetComponent<CanWalk>();

     if (speedUpgrade)
         AddSpeed ();

     if (canWalk.CanMove)
     {
         if (GameObject.FindGameObjectWithTag ("CodeInput").GetComponent<Text> ().text == "#SPDx2=true")
             cheat = true;

         if (GameObject.FindGameObjectWithTag ("CodeInput").GetComponent<Text> ().text == "#SPDx2=false")
             cheat = false;

         if (!invertVertical) {inputV = Input.GetAxis("Vertical");}
         if (invertVertical) {inputV = -Input.GetAxis("Vertical");}

         if (cheat) {
             if (!Input.GetKey (runButton) && !Input.GetKey (crowtchButton)) {
                 moveZ = inputV * speed * 2 * Time.fixedDeltaTime;
             }
             if (Input.GetKey (runButton) && !Input.GetKey (crowtchButton)) {
                 moveZ = inputV * speed * speedMultiplier * 2 * Time.fixedDeltaTime;
             }
             if (Input.GetKey (crowtchButton) && !Input.GetKey (runButton)) {
                 moveZ = inputV * speed / speedDivisor * 2 * Time.fixedDeltaTime;
             }
             if (Input.GetKey (crowtchButton) && Input.GetKey (runButton)) {
                 moveZ = inputV * speed * 2 * Time.fixedDeltaTime;
             }
         }

         else if (!cheat) {
             if (!Input.GetKey (runButton) && !Input.GetKey (crowtchButton)) {
                 moveZ = inputV * speed * Time.fixedDeltaTime;
             }
             if (Input.GetKey (runButton) && !Input.GetKey (crowtchButton)) {
                 moveZ = inputV * speed * speedMultiplier * Time.fixedDeltaTime;
             }
             if (Input.GetKey (crowtchButton) && !Input.GetKey (runButton)) {
                 moveZ = inputV * speed / speedDivisor * Time.fixedDeltaTime;
             }
             if (Input.GetKey (crowtchButton) && Input.GetKey (runButton)) {
                 moveZ = inputV * speed * Time.fixedDeltaTime;
             }
         }
         print (Time.timeScale.ToString ()); 

         rbody.velocity = new Vector3(0,rbody.velocity.y,0);

         if (inputV != 0)
         {rbody.AddForce(transform.forward * moveZ * Time.fixedDeltaTime, ForceMode.Force);}
     }
 }

 void AddSpeed ()
 {
     speed += 100;
     Save ();
     speedUpgrade = false;
 }

 public void Save ()
 {
     PlayerPrefs.SetFloat ("FBSpeed", speed);
 }

 public void Load ()
 {
     speed = PlayerPrefs.GetFloat ("FBSpeed", 12000f);
 }

The speed calcule is, basically one of three, with some variants:

moveZ = 1 x 12000f x 0.02; moveZ = 240;

moveZ = -1 x 12000f x 0.02; moveZ = -240;

moveZ = 0 x 12000f x 0.02; moveZ = 0;

I use a external script to access the AddSpeed void. Can't find where I am wrong. Help?