Unity APC game ("vehicle logic ") wheel collider insanely slow

hello, im having an issue with my Apc game( like tank but smaller ") in unity 5.4 . i've used lots of scripts with the wheel collider, but i move very very slow. here my script :

 #pragma strict
 var centerOfMass : Vector3;
 var wheelFL : WheelCollider;
 var wheelFR : WheelCollider;
 var wheelRL : WheelCollider;
 var wheelRR : WheelCollider;
 
 var heighestSpeed : float = 250;
 var lowSpeedSteerAngel : float = 35 ;
 var highSpeedSteerAngel : float = 35;
 var decellarationSpeed : float = 2500 ;
 var maxTorque : float  = 2500;
 
  function Start(){
   GetComponent.<Rigidbody>().centerOfMass.y = -0.9;
  }
 
 
 function FixedUpdate()
 {
 Controle ();
 
 }
 
 function Update (){
  
 }
 
 function Controle (){
 
 wheelRR.motorTorque = maxTorque * Input.GetAxis("Vertical");
 wheelRL.motorTorque = maxTorque * Input.GetAxis("Vertical");
  if (Input.GetButton ("Vertical") == false) {
  wheelRR.brakeTorque = decellarationSpeed;
   wheelRL.brakeTorque = decellarationSpeed;
 
  }
  else {
  wheelRR.brakeTorque = 0;
   wheelRL.brakeTorque = 0;
  }
 var speedFactor = GetComponent.<Rigidbody>().velocity.magnitude/heighestSpeed;
 var currentSteerAngel = Mathf.Lerp(lowSpeedSteerAngel,highSpeedSteerAngel,speedFactor );
  currentSteerAngel*= Input.GetAxis("Horizontal");
 wheelFL.steerAngle = currentSteerAngel;
 wheelFR.steerAngle = currentSteerAngel;
 
 }
 
 

i know that unity wheel collider has change but i really need some help !!!