How do i add my objects to a tag

I've made this "frogger" game with 8 cars, but i want to have an "up" and a "down" tag so i dont have to make a startcar1-100 for every car. I already made the tags but i dont know how to add the objects to the tag

 using UnityEngine;
 using System.Collections;
 
 public class scriptCar : MonoBehaviour {
 
     public GameObject up, down, startCar1, startCar2, startCar3, startCar4, startCar5, startCar6, startCar7, startCar8;
     public float speed1, speed2, speed3, speed4, speed5, speed6, speed7, speed8;
     public int score;
 
     // Use this for initialization
     void Start () {
 
     }
     
     // Update is called once per frame
     void Update () {
         
         speed1 = Random.Range (5, 10);
         speed2 = Random.Range (7, 12);
         speed3 = Random.Range (1, 6);
         speed4 = Random.Range (9, 14);
         speed5 = Random.Range (4, 9);
         speed6 = Random.Range (6, 1);
         speed7 = Random.Range (7, 12);
         speed8 = Random.Range (3, 9);
 
         startCar1.transform.Translate (0, -speed2 * Time.deltaTime, 0);    
         if (startCar1.transform.position.y < -9){
             startCar1.transform.position = new Vector3 (-2.3F, 8, 0);
             
         }
 
         startCar2.transform.Translate (0, -speed2 * Time.deltaTime, 0);        
         if (startCar2.transform.position.y < -9){
             startCar2.transform.position = new Vector3 (-1.65F, 8, 0);
         }
 
         startCar3.transform.Translate (0, -speed3 * Time.deltaTime, 0);        
         if (startCar3.transform.position.y < -9){
             startCar3.transform.position = new Vector3 (-1.05F, 8, 0);
         }
 
         startCar4.transform.Translate (0, -speed4 * Time.deltaTime, 0);        
         if (startCar4.transform.position.y < -9){
             startCar4.transform.position = new Vector3 (-0.35F, 8, 0);
         }
 
         startCar5.transform.Translate (0, speed5 * Time.deltaTime, 0);        
         if (startCar5.transform.position.y > 9){
             startCar5.transform.position = new Vector3 (0.35F, -8, 0);
         }
 
         startCar6.transform.Translate (0, speed6 * Time.deltaTime, 0);        
         if (startCar6.transform.position.y > 9){
             startCar6.transform.position = new Vector3 (1F, -8, 0);
         }
 
         startCar7.transform.Translate (0, speed7 * Time.deltaTime, 0);        
         if (startCar7.transform.position.y > 9){
             startCar7.transform.position = new Vector3 (1.65F, -8, 0);
         }
 
         startCar8.transform.Translate (0, speed8 * Time.deltaTime, 0);        
         if (startCar8.transform.position.y > 9){
             startCar8.transform.position = new Vector3 (2.25F, -8, 0);
         }
         
     }
 }