how to track object movement speed 2d

sisse008 has already posted what should be the correct answer, but in case you weren't sure how to estimate speed manually:

 using UnityEngine;
 using System.Collections;
 
 public class CheckMySpeed : MonoBehaviour
 {
     private Vector3 previousPosition;
     private float estimatedSpeed;
 
     // Take an initial value for our position.  The first value might end up
     // being zero, but this tends to happen with numerical approximations.
     void Awake ()
     {
         this.previousPosition = this.transform.position;
     }
 
     // It sounded like you were using physics on your objects, so we'll
     // check the speed in the physics update
     void FixedUpdate ()
     {
         // "Speed" is defined as the change in position divided by the change in time.
         float delta = (this.transform.position - previousPosition).magnitude;
         this.estimatedSpeed = delta / Time.fixedDeltaTime;
 
         // Confirm it's working
         Debug.LogFormat ("CheckMySpeed.estimatedSpeed: {0}", this.estimatedSpeed);
     }
 }