object rotates when moving backward

whenever i press s to move my vehicle backwards it also rotates it to the right, this is a script i made ages ago when i was new to unity and i cant see anything wrong.

 var weapon = 0;
 var m = 1;
 var f = 2;
 var c = 3;
 var machinegun1 : GameObject;
 var machinegun2 : GameObject;
 var flamethrower : GameObject;
 var cannon: GameObject;
 var tractor1cube : GameObject;
 var forwards : float = 0;
 var turn : float = 0;
 var maxSpeed = 8;
 var minSpeed = -6;
 var maxTurn = 135;
 
 function Update () {
 maxTurn = forwards / 6 * 135 + 20;
 if (weapon >= 1)
 Invoke("WeaponSelect",0);
 
 if (weapon >= 1){
  transform.Rotate(Vector3.up * Time.deltaTime * turn);
  }
 
 if (weapon >= 1){
   transform.Translate(Vector3.left * Time.deltaTime * forwards);
   }
   
   if(turn >= 0){
   turn -= 3.25;
   }
   
   if(turn <= 0){
   turn += 3.25;
   }
   
   if(forwards <= 0){
   forwards += 0.2;
   }
   
   if(forwards >= 0){
   forwards -= 0.1;
   }
   
   if(forwards >= maxSpeed){
   forwards = maxSpeed;
   }
   
   if(forwards <= minSpeed){
   forwards = minSpeed;
   }
   
   if(turn >= maxTurn){
   turn = maxTurn;
   }
   
   if(turn <= -maxTurn){
   turn = -maxTurn;
   }
 
 if (Input.GetKey("a")) {
 turn -= 7.5;
 if(forwards >= maxSpeed / 2){
 transform.Translate(Vector3.forward * Time.deltaTime * turn / 50);
 }
 if(forwards >= maxSpeed * 0.75){
 forwards -= 0.6;
 }
 }
 
 if(Input.GetKey("d")) {
 turn += 7.5;
 if(forwards >= maxSpeed / 2){
 transform.Translate(Vector3.forward * Time.deltaTime * -turn / 50);
 }
 if(forwards >= maxSpeed * 0.75){
 forwards -= 0.6;
 }
 }
 
 if (Input.GetKey("w")) {
 forwards += 0.3;
 }
 
 if (Input.GetKey("s")) {
 forwards -= 0.5;
 }
 }
 
 function OnGUI () {
     if (weapon < 1){
     if (GUI.Button (Rect (Screen.width * 0.02,60,160,120), "Guns")) {
         weapon = m;
         Destroy (tractor1cube);
         }
         }
         if (weapon < 1){
     if (GUI.Button (Rect (Screen.width * 0.02,220,160,120), "Flamethrower")) {
         weapon = f;
         Destroy (tractor1cube);
     }
     }
     if (weapon < 1){
     if (GUI.Button (Rect (Screen.width * 0.02,380,160,120), "Cannon")) {
         weapon = c;
         Destroy (tractor1cube);
     }
     }
 }
 
 function WeaponSelect() { 
 if (weapon != m)
 Destroy (machinegun1);
 
 if (weapon != m)
 Destroy (machinegun2);
 
 if (weapon != f)
 Destroy (flamethrower);
 
 if (weapon != c)
 Destroy (cannon);
 }

also with the

 if(forwards >= maxSpeed / 2){
     transform.Translate(Vector3.forward * Time.deltaTime * turn / 50);
     }

part if i rotate it right it drifts around a point in front of it but if i rotate left it drifts around a point behind it, does anyone know why or how to fix it?