Best way to get variable from another script

I'm working on a 2D game and I have many c# script. Some variable like Speed influence to a lot of gameObject's and script. I want to all of these script's read Speed variable from just one script and I do this with property and it worked very well.

But my problem is I want when I change the Speed from that script, Other script detect this change and change their own Speed variable. Currently it's not like this because those script read Speed in Start() method. I have some idea to solve this problem but I think these are not good:

• Get variable from that component in Update() or FixedUpdate() method (I think it's bad because every frame this code runs)

• In that public script I create a list of script's that use Speed variable and when Speed changed, send a message to those script's.

• And I have a lot of other idea that I know it's wrong so I don't write that.

Update:

This is my public class that contain Speed variable:

 using UnityEngine;
 using System.Collections;

 public class Utility : MonoBehaviour {

     public float _Speed = 2f;
     public float Speed
     {
         get{return _Speed;}
         set{_Speed = value;}
     }

     void Start () {
     }
 

     void Update () {
         //Here I can detect change of this variable and send message to other scripts
     }
 }

And other script access to that variable like this( Just example):

 void Start () {
         float mySpeed = GetComponent<Utility> ().Speed;
 }
 void Update () {
     // Or here I can read speed every frame like below:
     // mySpeed = GetComponent<Utility> ().Speed;
 }