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.