Add more gravity to car

Unity Engine
1

i have a car and he drives nice and stable.
the only problem is when he is jump over a little hill.
the car need to go faster back to the terrain.

i have following script attached to my car but no of my options works for me:

var fly = false;

function  OnCollisionEnter  (collisionInfo : Collision) {
 if (collisionInfo.gameObject.tag == "Terrain"){
 	fly = false;
 }
}

function OnCollisionExit(collisionInfo : Collision) {
   if (collisionInfo.gameObject.tag == "Terrain"){
 	fly = true;
 }
}

function Update() {
	if (fly==true) {

// i tried this options
		transform.position.y -= 30 * Time.deltaTime;
		transform.position.y -= 1;
		rigidbody.AddForce (0, -10, 0);
	}
}
2

Have you tried simply adding more gravity to the physics? Edit/Project Settings/Physics then the gravity vector

3

yeah i have tried this. but than the reaction of the car is very bad.