Speedometer Error. PLS help (((

this is my car script, where have been put speedometer script with this tutorial http://answers.unity3d.com/questions/15414/how-can-i-make-an-on-screen-speedometer.html, but when i put this

 var mph = rigidbody.velocity.magnitude * 2.237;
 mphDisplay.text = mph + " MPH"; 

i get error: UnassignedReferenceException: The variable mphDisplay of 'Car' has not been assigned. You probably need to assign the mphDisplay variable of the Car script in the inspector. Car.Update () (at Assets/Scripts/JavaScripts/Car.js:90)

please help, this car scipt is made by cartutorial, sorry for my bad english.

 private var wheelRadius : float = 0.4;
 var suspensionRange : float = 0.1;
 var suspensionDamper : float = 50;
 var suspensionSpringFront : float = 18500;
 var suspensionSpringRear : float = 9000;
 
 public var brakeLights : Material;
 
 var dragMultiplier : Vector3 = new Vector3(2, 5, 1);
 var mphDisplay : GUIText;
 var throttle : float = 0; 
 private var steer : float = 0;
 private var handbrake : boolean = false;
 
 var centerOfMass : Transform;
 
 var frontWheels : Transform[];
 var rearWheels : Transform[];
 
 private var wheels : Wheel[];
 wheels = new Wheel[frontWheels.Length + rearWheels.Length];
 
 private var wfc : WheelFrictionCurve;
 
 var topSpeed : float = 160;
 var numberOfGears : int = 5;
 
 var maximumTurn : int = 15;
 var minimumTurn : int = 10;
 
 var resetTime : float = 5.0;
 private var resetTimer : float = 0.0;
 
 private var engineForceValues : float[];
 private var gearSpeeds : float[];
 
 private var currentGear : int;
 private var currentEnginePower : float = 0.0;
 
 private var handbrakeXDragFactor : float = 0.5;
 private var initialDragMultiplierX : float = 10.0;
 private var handbrakeTime : float = 0.0;
 private var handbrakeTimer : float = 1.0;
 
 private var skidmarks : Skidmarks = null;
 private var skidSmoke : ParticleEmitter = null;
 var skidmarkTime : float[];
 
 private var sound : SoundController = null;
 sound = transform.GetComponent(SoundController);
 
 private var canSteer : boolean;
 private var canDrive : boolean;
 
 class Wheel
 {
  var collider : WheelCollider;
  var wheelGraphic : Transform;
  var tireGraphic : Transform;
  var driveWheel : boolean = false;
  var steerWheel : boolean = false;
  var lastSkidmark : int = -1;
  var lastEmitPosition : Vector3 = Vector3.zero;
  var lastEmitTime : float = Time.time;
  var wheelVelo : Vector3 = Vector3.zero;
  var groundSpeed : Vector3 = Vector3.zero;
 }
 
 function Start()
 { 
  // Measuring 1 - 60
  accelerationTimer = Time.time;
  
  SetupWheelColliders();
  
  SetupCenterOfMass();
  
  topSpeed = Convert_Miles_Per_Hour_To_Meters_Per_Second(topSpeed);
  
  SetupGears();
  
  SetUpSkidmarks();
  
  initialDragMultiplierX = dragMultiplier.x;
 }
 
 function Update()
 { 
  var mph = rigidbody.velocity.magnitude * 2.237;
  mphDisplay.text = mph + " MPH";
  var relativeVelocity : Vector3 = transform.InverseTransformDirection(rigidbody.velocity);
  
  GetInput();
  
  Check_If_Car_Is_Flipped();
  
  UpdateWheelGraphics(relativeVelocity);
  
  UpdateGear(relativeVelocity);
 }
 
 function FixedUpdate()
 { 
  // The rigidbody velocity is always given in world space, but in order to work in local space of the car model we need to transform it first.
  var relativeVelocity : Vector3 = transform.InverseTransformDirection(rigidbody.velocity);
  
  CalculateState(); 
  
  UpdateFriction(relativeVelocity);
  
  UpdateDrag(relativeVelocity);
  
  CalculateEnginePower(relativeVelocity);
  
  ApplyThrottle(canDrive, relativeVelocity);
  
  ApplySteering(canSteer, relativeVelocity);
 }
 
 /**************************************************/
 /* Functions called from Start()                  */
 /**************************************************/
 
 function SetupWheelColliders()
 {
  SetupWheelFrictionCurve();
  
  var wheelCount : int = 0;
  
  for (var t : Transform in frontWheels)
  {
  wheels[wheelCount] = SetupWheel(t, true);
  wheelCount++;
  }
  
  for (var t : Transform in rearWheels)
  {
  wheels[wheelCount] = SetupWheel(t, false);
  wheelCount++;
  }
 }
 
 function SetupWheelFrictionCurve()
 {
  wfc = new WheelFrictionCurve();
  wfc.extremumSlip = 0;
  wfc.extremumValue = 25;
  wfc.asymptoteSlip = 0;
  wfc.asymptoteValue = 12;
  wfc.stiffness = 1;
 }
 
 function SetupWheel(wheelTransform : Transform, isFrontWheel : boolean)
 {
  var go : GameObject = new GameObject(wheelTransform.name + " Collider");
  go.transform.position = wheelTransform.position;
  go.transform.parent = transform;
  go.transform.rotation = wheelTransform.rotation;
  
  var wc : WheelCollider = go.AddComponent(typeof(WheelCollider)) as WheelCollider;
  wc.suspensionDistance = suspensionRange;
  var js : JointSpring = wc.suspensionSpring;
  
  if (isFrontWheel)
  js.spring = suspensionSpringFront;
  else
  js.spring = suspensionSpringRear;
  
  js.damper = suspensionDamper;
  wc.suspensionSpring = js;
  
  wheel = new Wheel(); 
  wheel.collider = wc;
  wc.sidewaysFriction = wfc;
  wheel.wheelGraphic = wheelTransform;
  wheel.tireGraphic = wheelTransform.GetComponentsInChildren(Transform)[1];
  
  wheelRadius = wheel.tireGraphic.renderer.bounds.size.y / 2; 
  wheel.collider.radius = wheelRadius;
  
  if (isFrontWheel)
  {
  wheel.steerWheel = true;
  
  go = new GameObject(wheelTransform.name + " Steer Column");
  go.transform.position = wheelTransform.position;
  go.transform.rotation = wheelTransform.rotation;
  go.transform.parent = transform;
  wheelTransform.parent = go.transform;
  }
  else
  wheel.driveWheel = true;
  
  return wheel;
 }
 
 function SetupCenterOfMass()
 {
  if(centerOfMass != null)
  rigidbody.centerOfMass = centerOfMass.localPosition;
 }
 
 function SetupGears()
 {
  engineForceValues = new float[numberOfGears];
  gearSpeeds = new float[numberOfGears];
  
  var tempTopSpeed : float = topSpeed;
  
  for(var i = 0; i < numberOfGears; i++)
  {
  if(i > 0)
  gearSpeeds[i] = tempTopSpeed / 4 + gearSpeeds[i-1];
  else
  gearSpeeds[i] = tempTopSpeed / 4;
  
  tempTopSpeed -= tempTopSpeed / 4;
  }
  
  var engineFactor : float = topSpeed / gearSpeeds[gearSpeeds.Length - 1];
  
  for(i = 0; i < numberOfGears; i++)
  {
  var maxLinearDrag : float = gearSpeeds[i] * gearSpeeds[i];// * dragMultiplier.z;
  engineForceValues[i] = maxLinearDrag * engineFactor;
  }
 }
 
 function SetUpSkidmarks()
 {
  if(FindObjectOfType(Skidmarks))
  {
  skidmarks = FindObjectOfType(Skidmarks);
  skidSmoke = skidmarks.GetComponentInChildren(ParticleEmitter);
  }
  else
  Debug.Log("No skidmarks object found. Skidmarks will not be drawn");
  
  skidmarkTime = new float[4];
  for (var f : float in skidmarkTime)
  f = 0.0;
 }
 
 /**************************************************/
 /* Functions called from Update()                 */
 /**************************************************/
 
 function GetInput()
 {
  throttle = Input.GetAxis("Vertical");
  steer = Input.GetAxis("Horizontal");
  
  if(throttle < 0.0)
  brakeLights.SetFloat("_Intensity", Mathf.Abs(throttle));
  else
  brakeLights.SetFloat("_Intensity", 0.0);
  
  CheckHandbrake();
 }
 
 function CheckHandbrake()
 {
  if(Input.GetKey("space"))
  {
  if(!handbrake)
  {
  handbrake = true;
  handbrakeTime = Time.time;
  dragMultiplier.x = initialDragMultiplierX * handbrakeXDragFactor;
  }
  }
  else if(handbrake)
  {
  handbrake = false;
  StartCoroutine(StopHandbraking(Mathf.Min(5, Time.time - handbrakeTime)));
  }
 }
 
 function StopHandbraking(seconds : float)
 {
  var diff : float = initialDragMultiplierX - dragMultiplier.x;
  handbrakeTimer = 1;
  
  // Get the x value of the dragMultiplier back to its initial value in the specified time.
  while(dragMultiplier.x < initialDragMultiplierX && !handbrake)
  {
  dragMultiplier.x += diff * (Time.deltaTime / seconds);
  handbrakeTimer -= Time.deltaTime / seconds;
  yield;
  }
  
  dragMultiplier.x = initialDragMultiplierX;
  handbrakeTimer = 0;
 }
 
 function Check_If_Car_Is_Flipped()
 {
  if(transform.localEulerAngles.z > 80 && transform.localEulerAngles.z < 280)
  resetTimer += Time.deltaTime;
  else
  resetTimer = 0;
  
  if(resetTimer > resetTime)
  FlipCar();
 }
 
 function FlipCar()
 {
  transform.rotation = Quaternion.LookRotation(transform.forward);
  transform.position += Vector3.up * 0.5;
  rigidbody.velocity = Vector3.zero;
  rigidbody.angularVelocity = Vector3.zero;
  resetTimer = 0;
  currentEnginePower = 0;
 }
 
 var wheelCount : float;
 function UpdateWheelGraphics(relativeVelocity : Vector3)
 {
  wheelCount = -1;
  
  for(var w : Wheel in wheels)
  {
  wheelCount++;
  var wheel : WheelCollider = w.collider;
  var wh : WheelHit = new WheelHit();
  
  // First we get the velocity at the point where the wheel meets the ground, if the wheel is touching the ground
  if(wheel.GetGroundHit(wh))
  {
  w.wheelGraphic.localPosition = wheel.transform.up * (wheelRadius + wheel.transform.InverseTransformPoint(wh.point).y);
  w.wheelVelo = rigidbody.GetPointVelocity(wh.point);
  w.groundSpeed = w.wheelGraphic.InverseTransformDirection(w.wheelVelo);
  
  // Code to handle skidmark drawing. Not covered in the tutorial
  if(skidmarks)
  {
  if(skidmarkTime[wheelCount] < 0.02 && w.lastSkidmark != -1)
  {
  skidmarkTime[wheelCount] += Time.deltaTime;
  }
  else
  {
  var dt : float = skidmarkTime[wheelCount] == 0.0 ? Time.deltaTime : skidmarkTime[wheelCount];
  skidmarkTime[wheelCount] = 0.0;
 
  var handbrakeSkidding : float = handbrake && w.driveWheel ? w.wheelVelo.magnitude * 0.3 : 0;
  var skidGroundSpeed = Mathf.Abs(w.groundSpeed.x) - 15;
  if(skidGroundSpeed > 0 || handbrakeSkidding > 0)
  {
  var staticVel : Vector3 = transform.TransformDirection(skidSmoke.localVelocity) + skidSmoke.worldVelocity;
  if(w.lastSkidmark != -1)
  {
  var emission : float = UnityEngine.Random.Range(skidSmoke.minEmission, skidSmoke.maxEmission);
  var lastParticleCount : float = w.lastEmitTime * emission;
  var currentParticleCount : float = Time.time * emission;
  var noOfParticles : int = Mathf.CeilToInt(currentParticleCount) - Mathf.CeilToInt(lastParticleCount);
  var lastParticle : int = Mathf.CeilToInt(lastParticleCount);
  
  for(var i = 0; i <= noOfParticles; i++)
  {
  var particleTime : float = Mathf.InverseLerp(lastParticleCount, currentParticleCount, lastParticle + i);
  skidSmoke.Emit( Vector3.Lerp(w.lastEmitPosition, wh.point, particleTime) + new Vector3(Random.Range(-0.1, 0.1), Random.Range(-0.1, 0.1), Random.Range(-0.1, 0.1)), staticVel + (w.wheelVelo * 0.05), Random.Range(skidSmoke.minSize, skidSmoke.maxSize) * Mathf.Clamp(skidGroundSpeed * 0.1,0.5,1), Random.Range(skidSmoke.minEnergy, skidSmoke.maxEnergy), Color.white);
  }
  }
  else
  {
  skidSmoke.Emit( wh.point + new Vector3(Random.Range(-0.1, 0.1), Random.Range(-0.1, 0.1), Random.Range(-0.1, 0.1)), staticVel + (w.wheelVelo * 0.05), Random.Range(skidSmoke.minSize, skidSmoke.maxSize) * Mathf.Clamp(skidGroundSpeed * 0.1,0.5,1), Random.Range(skidSmoke.minEnergy, skidSmoke.maxEnergy), Color.white);
  }
  
  w.lastEmitPosition = wh.point;
  w.lastEmitTime = Time.time;
  
  w.lastSkidmark = skidmarks.AddSkidMark(wh.point + rigidbody.velocity * dt, wh.normal, (skidGroundSpeed * 0.1 + handbrakeSkidding) * Mathf.Clamp01(wh.force / wheel.suspensionSpring.spring), w.lastSkidmark);
  sound.Skid(true, Mathf.Clamp01(skidGroundSpeed * 0.001));
  }
  else
  {
  w.lastSkidmark = -1;
  sound.Skid(false, 0);
  }
  }
  }
  }
  else
  {
  // If the wheel is not touching the ground we set the position of the wheel graphics to
  // the wheel's transform position + the range of the suspension.
  w.wheelGraphic.position = wheel.transform.position + (-wheel.transform.up * suspensionRange);
  if(w.steerWheel)
  w.wheelVelo *= 0.9;
  else
  w.wheelVelo *= 0.9 * (1 - throttle);
  
  if(skidmarks)
  {
  w.lastSkidmark = -1;
  sound.Skid(false, 0);
  }
  }
  // If the wheel is a steer wheel we apply two rotations:
  // *Rotation around the Steer Column (visualizes the steer direction)
  // *Rotation that visualizes the speed
  if(w.steerWheel)
  {
  var ea : Vector3 = w.wheelGraphic.parent.localEulerAngles;
  ea.y = steer * maximumTurn;
  w.wheelGraphic.parent.localEulerAngles = ea;
  w.tireGraphic.Rotate(Vector3.right * (w.groundSpeed.z / wheelRadius) * Time.deltaTime * Mathf.Rad2Deg);
  }
  else if(!handbrake && w.driveWheel)
  {
  // If the wheel is a drive wheel it only gets the rotation that visualizes speed.
  // If we are hand braking we don't rotate it.
  w.tireGraphic.Rotate(Vector3.right * (w.groundSpeed.z / wheelRadius) * Time.deltaTime * Mathf.Rad2Deg);
  }
  }
 }
 
 function UpdateGear(relativeVelocity : Vector3)
 {
  currentGear = 0;
  for(var i = 0; i < numberOfGears - 1; i++)
  {
  if(relativeVelocity.z > gearSpeeds[i])
  currentGear = i + 1;
  }
 }
 
 /**************************************************/
 /* Functions called from FixedUpdate()            */
 /**************************************************/
 
 function UpdateDrag(relativeVelocity : Vector3)
 {
  var relativeDrag : Vector3 = new Vector3( -relativeVelocity.x * Mathf.Abs(relativeVelocity.x), 
  -relativeVelocity.y * Mathf.Abs(relativeVelocity.y), 
  -relativeVelocity.z * Mathf.Abs(relativeVelocity.z) );
  
  var drag = Vector3.Scale(dragMultiplier, relativeDrag);
  
  if(initialDragMultiplierX > dragMultiplier.x) // Handbrake code
  { 
  drag.x /= (relativeVelocity.magnitude / (topSpeed / ( 1 + 2 * handbrakeXDragFactor ) ) );
  drag.z *= (1 + Mathf.Abs(Vector3.Dot(rigidbody.velocity.normalized, transform.forward)));
  drag += rigidbody.velocity * Mathf.Clamp01(rigidbody.velocity.magnitude / topSpeed);
  }
  else // No handbrake
  {
  drag.x *= topSpeed / relativeVelocity.magnitude;
  }
  
  if(Mathf.Abs(relativeVelocity.x) < 5 && !handbrake)
  drag.x = -relativeVelocity.x * dragMultiplier.x;
  
 
  rigidbody.AddForce(transform.TransformDirection(drag) * rigidbody.mass * Time.deltaTime);
 }
 
 function UpdateFriction(relativeVelocity : Vector3)
 {
  var sqrVel : float = relativeVelocity.x * relativeVelocity.x;
  
  // Add extra sideways friction based on the car's turning velocity to avoid slipping
  wfc.extremumValue = Mathf.Clamp(300 - sqrVel, 0, 300);
  wfc.asymptoteValue = Mathf.Clamp(150 - (sqrVel / 2), 0, 150);
  
  for(var w : Wheel in wheels)
  {
  w.collider.sidewaysFriction = wfc;
  w.collider.forwardFriction = wfc;
  }
 }
 
 function CalculateEnginePower(relativeVelocity : Vector3)
 {
  if(throttle == 0)
  {
  currentEnginePower -= Time.deltaTime * 200;
  }
  else if( HaveTheSameSign(relativeVelocity.z, throttle) )
  {
  normPower = (currentEnginePower / engineForceValues[engineForceValues.Length - 1]) * 2;
  currentEnginePower += Time.deltaTime * 200 * EvaluateNormPower(normPower);
  }
  else
  {
  currentEnginePower -= Time.deltaTime * 300;
  }
  
  if(currentGear == 0)
  currentEnginePower = Mathf.Clamp(currentEnginePower, 0, engineForceValues[0]);
  else
  currentEnginePower = Mathf.Clamp(currentEnginePower, engineForceValues[currentGear - 1], engineForceValues[currentGear]);
 }
 
 function CalculateState()
 {
  canDrive = false;
  canSteer = false;
  
  for(var w : Wheel in wheels)
  {
  if(w.collider.isGrounded)
  {
  if(w.steerWheel)
  canSteer = true;
  if(w.driveWheel)
  canDrive = true;
  }
  }
 }
 
 function ApplyThrottle(canDrive : boolean, relativeVelocity : Vector3)
 {
  if(canDrive)
  {
  var throttleForce : float = 0;
  var brakeForce : float = 0;
  
  if (HaveTheSameSign(relativeVelocity.z, throttle))
  {
  if (!handbrake)
  throttleForce = Mathf.Sign(throttle) * currentEnginePower * rigidbody.mass;
  }
  else
  brakeForce = Mathf.Sign(throttle) * engineForceValues[0] * rigidbody.mass;
  
  rigidbody.AddForce(transform.forward * Time.deltaTime * (throttleForce + brakeForce));
  }
 }
 
 function ApplySteering(canSteer : boolean, relativeVelocity : Vector3)
 {
  if(canSteer)
  {
  var turnRadius : float = 3.0 / Mathf.Sin((90 - (steer * 30)) * Mathf.Deg2Rad);
  var minMaxTurn : float = EvaluateSpeedToTurn(rigidbody.velocity.magnitude);
  var turnSpeed : float = Mathf.Clamp(relativeVelocity.z / turnRadius, -minMaxTurn / 10, minMaxTurn / 10);
  
  transform.RotateAround( transform.position + transform.right * turnRadius * steer, 
  transform.up, 
  turnSpeed * Mathf.Rad2Deg * Time.deltaTime * steer);
  
  var debugStartPoint = transform.position + transform.right * turnRadius * steer;
  var debugEndPoint = debugStartPoint + Vector3.up * 5;
  
  Debug.DrawLine(debugStartPoint, debugEndPoint, Color.red);
  
  if(initialDragMultiplierX > dragMultiplier.x) // Handbrake
  {
  var rotationDirection : float = Mathf.Sign(steer); // rotationDirection is -1 or 1 by default, depending on steering
  if(steer == 0)
  {
  if(rigidbody.angularVelocity.y < 1) // If we are not steering and we are handbraking and not rotating fast, we apply a random rotationDirection
  rotationDirection = Random.Range(-1.0, 1.0);
  else
  rotationDirection = rigidbody.angularVelocity.y; // If we are rotating fast we are applying that rotation to the car
  }
  // -- Finally we apply this rotation around a point between the cars front wheels.
  transform.RotateAround( transform.TransformPoint( ( frontWheels[0].localPosition + frontWheels[1].localPosition) * 0.5), 
  transform.up, 
  rigidbody.velocity.magnitude * Mathf.Clamp01(1 - rigidbody.velocity.magnitude / topSpeed) * rotationDirection * Time.deltaTime * 2);
  }
  }
 }
 
 /**************************************************/
 /*               Utility Functions                */
 /**************************************************/
 
 function Convert_Miles_Per_Hour_To_Meters_Per_Second(value : float) : float
 {
  return value * 0.44704;
 }
 
 function Convert_Meters_Per_Second_To_Miles_Per_Hour(value : float) : float
 {
  return value * 2.23693629; 
 }
 
 function HaveTheSameSign(first : float, second : float) : boolean
 {
  if (Mathf.Sign(first) == Mathf.Sign(second))
  return true;
  else
  return false;
 }
 
 function EvaluateSpeedToTurn(speed : float)
 {
  if(speed > topSpeed / 2)
  return minimumTurn;
  
  var speedIndex : float = 1 - (speed / (topSpeed / 2));
  return minimumTurn + speedIndex * (maximumTurn - minimumTurn);
 }
 
 function EvaluateNormPower(normPower : float)
 {
  if(normPower < 1)
  return 10 - normPower * 9;
  else
  return 1.9 - normPower * 0.9;
 }
 
 function GetGearState()
 {
  var relativeVelocity : Vector3 = transform.InverseTransformDirection(rigidbody.velocity);
  var lowLimit : float = (currentGear == 0 ? 0 : gearSpeeds[currentGear-1]);
  return (relativeVelocity.z - lowLimit) / (gearSpeeds[currentGear - lowLimit]) * (1 - currentGear * 0.1) + currentGear * 0.1;
 }