make enemy cars spawn faster over time!

You need to think a bit about the game design first. There are several ways: you can look at the time remaining, the time spent on the level, or a fixed time step to modify the spawn rate.

so every 20 seconds you want reduce the timespan between spanw by 0.1 is that correct?(with max at 0.4)

You need to implement another time as the one already in you script but initializing at 20f I suggest you take more explicit names for the variables.

 //Global
 public float delayTimer2 = 20f;
  float Timer2;


  //In Update
  Timer2 -= Time.deltaTime;
  if (Timer2 <= 0)
  {
          if( delayTimer>0.5f){
              delayTimer -=0.1f
          }
       Timer2 = delayTimer2 ;
 }

this above code will go in update as well

It works but it goes directly to 0.5 i want to go from 0.8 to 0.7 to 0.6 and then 0.5 and i want this to happened every 20 seconds here is the script again:

 //Acceleration...
 public float delayTimer2 = 20f;
 float Timer2;
     


 public GameObject[] Cars;
 int carNo;
 public float delayTimer = 1f;
 float Timer;

 void Start () {

     


     Timer = delayTimer;
 
 }

 void Update () {

     Timer2 -= Time.deltaTime;
     Timer2 = delayTimer2;
     if(Timer2 <= 0)
     {
         if(delayTimer > 0.5f)
         {
             delayTimer -= 0.1f;
         }
     }


     Timer -= Time.deltaTime;
     if (Timer <= 0)
     {

         Vector3 carPos = new Vector3(Random.Range(-2.2f, 2.2f), transform.position.y, transform.position.z);
         carNo = Random.Range(0,5);
         Instantiate(Cars[carNo], carPos, transform.rotation);
         Timer = delayTimer;     
         
               
     }

}