Why isn't my integer changing when onTriggerEnter is called?

Hi,

I'm creating a game where you cross two lanes of traffic and I'm trying to get it so when my player collides with a certain car it tells me which car it was. I've tried making it so if it hits carA it results in the variable carCollision changing to 1 etc. But when the player collides with all cars in game it just produces '0'. What am I doing wrong?

public class CollisionCar : MonoBehaviour {

 void OnTriggerEnter(Collider col)
 {
     //Debug.Log(gameObject.name + "has collidied with" + col.gameObject.name);
     if (col.gameObject.tag == "carA")
     {
         mainManager.carCollision = 1;
     }
     if (col.gameObject.tag == "carB")
     {
         mainManager.carCollision = 2;
     }
     if (col.gameObject.tag == "carC")
     {
         mainManager.carCollision = 3;
     }
     if (col.gameObject.tag == "carD")
     {
         mainManager.carCollision = 4;
     }
     Debug.Log(mainManager.carCollision);
 }

}