how to clamp acceleration?

Hi guys, I need to give my vehicle an automatic acceleration up to a certain point, im pretty sure i need to use Mathf.Clamp but i’m not sure how to script it properly. The player doesnt control the speed, but my script currently has a user input to make it accelerate.

function FixedUpdate () {
//query input axes if necessarry
if(queryUserInput)
{
	brake = Mathf.Clamp01(-Input.GetAxis("Vertical"));
	//handbrake = Input.GetButton("Jump")?1.0:0.0;
	//steer = Input.GetAxis("Horizontal");
 	motor = Mathf.Clamp01(Input.GetAxis("Vertical"));
}
else
{
	motor = 0;
	//steer = 0;
	brake = 0;
	//handbrake = 0;
}

I need to change this motor into an automatic motor that never goes above a certain speed. If someone could help me out with the basic script I’ll of course edit the values myself, just need a start. Cheers - monster.

this link will definitely help u

http://unity3d.qatohost.com/questions/173673/car-accelerate-over-time-javascript-and-rigidbody.html