speedometer -not a GUI- but an actual gameobject on car dash -SOLVED!!!

this is a gameobject on my boats dashboard with this script on the needle. it rotates wonderfully initially as I accellerate, but doesn't go back down when I slow down. I tried adding a keyup ( for when I stop accellerating) and going in the other direction but it was a fail. any ideas?

 using UnityEngine;
 using System.Collections;
 namespace UnitySampleAssets.Vehicles.Car
 {
 public class speedOmeter : MonoBehaviour {
         public float speed ;
         MonoBehaviour boatSpeed;
         public GameObject boat;
         public Transform target;
         public float damping = 2f;
     void Awake () {
             boatSpeed = boat.GetComponent<boatBank>();
     }
 
     void Update () {
             speed = boatBank.boatSpeed;
 
                     transform.Rotate(0, 0, (-speed) * Time.deltaTime  );
 
 
     }
     }
 }

UPDATE*** seems it takes me to post here before I sort it out properly in my mind and come up with the answer. So here it is...I just don't care for a GUI speedometer because speed isn't really important but I wanted to have a working speedometer on the dash that simply rotates on it's Z axis to display speed - just to add life to the view

 using UnityEngine;
 using System.Collections;
 namespace UnitySampleAssets.Vehicles.Car
 {
 public class speedOmeter : MonoBehaviour {
         public float speed ;
         public bool accellerating;
         MonoBehaviour boatSpeed;
         public GameObject boat;
         
         public float damping = 2f;
     void Awake () {
             boatSpeed = boat.GetComponent<boatBank>();
     }
 
     void Update () {
             if (Input.GetKeyDown(KeyCode.W))
                 accellerating = true;
             if (Input.GetKeyUp(KeyCode.W))
                 accellerating = false;
             speed = boatBank.boatSpeed;
             if (accellerating == true)
                     transform.Rotate(0, 0, (-speed) * Time.deltaTime  );
             if (accellerating == false)
                 transform.Rotate(0, 0, (speed) * Time.deltaTime  );
 
 
     }
     }
 }