Rotate a parent object

So have four wheels on a car and the car body as separate objects that are grouped in the parent object car in my Hiearchy.

I need to access each wheel and the body separately so that I can have the player paint it.

The problem is that when I call transform.parent and try to rotate, the car rotates about a large radius and not about its center.

public Renderer carBody;
public Renderer carWheel1;
public Renderer carWheel2;
public Renderer carWheel3;
public Renderer carWheel4;

void Start ()
{
    carBody.material.color = Color.red;
    carWheel1.material.color = Color.black;
    carWheel2.material.color = Color.black;
    carWheel3.material.color = Color.black;
    carWheel4.material.color = Color.black;
}

// Update is called once per frame
void Update ()
{
    var rotationDirection = new Vector3(0, 1, 0);
        var carObject = carBody.transform.parent;
        carObject.transform.Rotate((rotationDirection * Time.deltaTime * rotationSpeed));
}

I need to know how to rotate the parent around its center and not around a larger radius of rotation.

Thank you in advance.

What is the parent of the carBody? Typically for rotation I will make the parent an empty game object and place it at the center of the car (i.e. the point I want to rotate around). If the parent needs to be off center, you can add an empty game object in between.