Sprinting and Tiredness

Hey guys,
I have a small issue, I would like my sprintSpeed variable to be proportional to the amount of energy my player has left, basically the more he sprints the slower he will sprint. I tried something like this, but it is not the desired effect:

var energy = 100.0;
var sprintSpeed = 12.0;

function Update ()
{	

	if(energy <= 100 && energy > 75)
	{
		sprintSpeed = 12.0;
	}
	if(energy <= 75 && energy > 50)
	{
		sprintSpeed = 10.0;
	}
	if(energy <= 50 && energy > 25)
	{
		sprintSpeed = 8.0;
	}
	if(energy <= 25 && energy >= 0)
	{
		sprintSpeed = 6.0;
	}
}

Any help is appreciated

var energy = 100.0;
var sprintSpeed = 12.0;

function Update ()
{

         sprintSpeed = 6.0+(0.06f*energy);

}