I have a Car, who needs a Box Collider. If the car would be 1 single mesh, i would only need to call:

        boxCol = gameObject.AddComponent<BoxCollider>();

and it would create a boxCollider, which fits perfectly my car, based on my car's mesh.

Now I have a car which exists of many different parts in the hierarchy. (for example: Body of the car is the parent of the 4 doors. Every door is 1 seperate gameobject, and has a doorknob child etc. )

now i need a script which changes the boxCollider so, that it has the box surrounding the whole car.

i found: http://answers.unity3d.com/questions/22019/auto-sizing-primitive-collider-based-on-child-mesh.html

but it just doesnt get me the right collider.

Edit: I just tried again - and the Boxcollider stays the same.

using UnityEngine;
using UnityEditor;
using System.Collections;

public class ColliderToFit : MonoBehaviour
{

    [MenuItem("My Tools/Collider/Fit to Children")]
    static void FitToChildren()
    {
        foreach (GameObject rootGameObject in Selection.gameObjects)
        {
            if (!(rootGameObject.GetComponent<Collider>()  is BoxCollider))
                continue;

            bool hasBounds = false;
            Bounds bounds = new Bounds(Vector3.zero, Vector3.zero);

            for (int i = 0; i < rootGameObject.transform.childCount; ++i)
            {
                Renderer childRenderer = rootGameObject.transform.GetChild(i).GetComponent<Renderer>();
                if (childRenderer != null)
                {
                    if (hasBounds)
                    {
                        bounds.Encapsulate(childRenderer.bounds);
                    }
                    else
                    {
                        bounds = childRenderer.bounds;
                        hasBounds = true;
                    }
                }
            }

            BoxCollider collider = (BoxCollider)rootGameObject.GetComponent<Collider>();
            collider.center = bounds.center - rootGameObject.transform.position;
            collider.size = bounds.size;
        }
    }

}