Rigidbody speed / Velocity issue

hii all ,

I am making game like : https://play.google.com/store/apps/details?id=com.egs.games.nobrakes

I have did code for moving and turning my player :

 void FixedUpdate () {
 
         if(Camera.main.orthographicSize == 18)
         {
             ON = true;
             trees.SetActive(true);
         }
 
         if(ON)
         {
             rigidbody.drag = 0.8f;
             rigidbody.angularDrag = 0.05f;
             score = Convert.ToInt32(velocity);
             speedTxt.text = velocity.ToString ();
             Movement();
             SpeedControl ();
         }
     }
 
     void Movement()
     {
         if(!gameOver)
         {
             rigidbody.AddForce (transform.forward * speed);
             //rigidbody.velocity = Vector3.ClampMagnitude (rigidbody.velocity, 100.0f);
             velocity = Mathf.Round (rigidbody.velocity.magnitude * 5.0f);
 
             if(Input.touchCount > 0) {
                 Input.multiTouchEnabled = true;
                 
                 for(var i = 0; i < Input.touchCount; i++)
                 {
                     if(Input.touches[i].phase==TouchPhase.Stationary || Input.touches[i].phase==TouchPhase.Began)
                     {        
                         Ray ray = Camera.main.ScreenPointToRay (Input.touches[i].position);
                         //Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
                         RaycastHit hit ;
 
                         
                         // Check what is under the  pointer
                         if (Physics.Raycast (ray, out hit, Mathf.Infinity)) 
                         {
 
                             if(hit.collider.name == "Left")
                             {
                                 yRotation -= 4.0f;
                                 transform.eulerAngles = new Vector3(0, yRotation, 0);
                                 //rigidbody.AddForce (-transform.right);
                                 rigidbody.AddForce (transform.forward * speed);
                                 rigidbody.angularDrag += 5.0f;
                                 rigidbody.drag += 1.0f;
                                 
                             }
                             
                             if(hit.collider.name == "Right")
                             {
                                 yRotation += 4.0f;
                                 transform.eulerAngles = new Vector3(0, yRotation, 0);
                                 //rigidbody.AddForce (transform.right);
                                 rigidbody.AddForce (transform.forward * speed);
                                 rigidbody.angularDrag += 5.0f;
                                 rigidbody.drag += 1.0f;
 
                             }
                         }
                     }
                 }
             }
         }
     }
 
     void SpeedControl()
     {
         if(Application.loadedLevelName == "1" || Application.loadedLevelName == "2" || Application.loadedLevelName == "3")
         {
             if(velocity <=105)
             {
                 speed += 0.004f;
             }
 
             if(velocity > 105 && velocity <=130)
             {
                 speed += 0.003f;
             }
 
             if(velocity > 130 && velocity <=190)
             {
                 speed += 0.002f;
             }
 
             if(velocity > 190 )
             {
                 speed += 0.001f;
             }
         }
 }

But while turning , speed is decreasing too much and then its not increasing fast so if my speed is 40 and if i drift then it will become arround 25 and then its not increasing fastly. So player keeps moving in between range of 20 - 50 if he continues to take turn.

Hope you guys help me.

Thanks..