Unity 5 Car WheelCollider Acceleration Issue

Recently I needed to make a small car game for a school project. However, after I setup the car and my simple car control script, the car drives fine, but keeps on accelerating after I release the up arrow key and keeps on slowing down after I release the down arrow key. How do I make it so that the car only accelerates only when the key is pressed down? I have tried to change the gravity and sensitivity in the InputManager, but it doesn't work. #pragma strict

 var wheelFL : WheelCollider;
 var wheelFR : WheelCollider;
 var wheelRL : WheelCollider;
 var wheelRR : WheelCollider;
 var steeringAngle : float;
 var throttle : float;
 var maxTorque : float;
 
 var speed : float;
 
 function Start () {
 
 }
 
 function Update () {
 
 throttle = Input.GetAxis("Vertical") * maxTorque;
 wheelRL.motorTorque = throttle;
 wheelRR.motorTorque = throttle;
 
 steeringAngle = Input.GetAxis("Horizontal") * 15;
 wheelFL.steerAngle = steeringAngle;
 wheelFR.steerAngle = steeringAngle;
 
 speed = Mathf.Round(GetComponent.<Rigidbody>().velocity.magnitude * 3.6);
 
 }