Move car at same speed

I want to move car on each game play run with same speed. At present I was getting each time different velocity. I want to fix that.

Here is game play area on which I was moving car:

At present I was moving car in infinite motion without any kind of control. When it collide with wall, it will its direction based on collision calculation.

Here is my source code that I was using to move car:

 void Start ()
     {
         direction = new Vector2 (Random.Range (-1f, 1f), Random.Range (-1f, 1f));
     }
 
     void FixedUpdate ()
     {
 
         float angle = Mathf.Atan2 (direction.y, direction.x) * Mathf.Rad2Deg;
         float targetRot = Mathf.LerpAngle (myRigidbody.rotation, (angle - 90f), 0.1f);
         myRigidbody.MoveRotation (targetRot);
 
         myRigidbody.velocity = direction * speed;
 //        myRigidbody.MovePosition (myRigidbody.position + direction * Time.deltaTime * speed);
 
         Debug.Log("vel mag: " + myRigidbody.velocity.magnitude);
     }
 
     void OnCollisionEnter2D (Collision2D other)
     {
         ContactPoint2D contact;
         contact = other.contacts [0];
 
 //        direction = 2 * (Vector3.Dot(direction, Vector3.Normalize(contact.normal))) * Vector3.Normalize(contact.normal) - direction; //Following formula  v' = 2 * (v . n) * n - v
 //
 //        direction *= -1;// Dont know why I had to multiply by -1 but it's inversed otherwisae
         Vector2 reflectedVelocity = Vector2.Reflect(direction, contact.normal);   
         direction = reflectedVelocity;
     }

At present on each run of car, I was getting different velocity that I have checked via debug statement. So my target is to move car with same speed always.