Make String Update for Value (GUI)

I’m making a space sim, and I need the speed bar to show how fast you’re going. My variable for the speed you’re going at is currentSpeed, but when I right that in the string part, it doesn’t show the current speed. I knew that was going to happen, so how should I make the string show the value of a float?

var currentSpeed : float;

function OnGUI()
	GUI.backgroundColor =;
	GUI.contentColor = Color.white; = Skin;
    GUI.backgroundColor = Color.yellow;
	GUI.contentColor = Color.white; = Skin;
    GUI.Button(Rect(10,670,currentSpeed * 5,20), "currentSpeed");

As you can see, currentSpeed is a string, and won’t be the float variable. Is there away for the string to show the value of currentSpeed?