If you want something to be attached to a gameobject and don’t want to keep track of the mycar variable yourself it might be worthwhile to use the Car script as a component and then add that to mycar. Make Car as a separate script and then use AddComponent and GetComponent.
//Instantiate some prefab
prefab.AddComponent(Car);
prefab.name = "RacingCar";
// then if you want to get the component
GameObject theCar = GameObject.Find("RacingCar");
var carScript : Car = theCar.GetComponent(Car);
Check out the Component.GetComponent part of the manual for more details.