In Box2d you would create your car body and wheels, and join the wheels to the body, then add the whole thing to the box2d World.
Then replace the code you have - where you set the position of the various parts, instead you query the position of the box2d objects, translate them to screen coordinates, then set your sprite's position.
The easiest way to start is to create your box2d world and simple vehicle, make sure you are displaying debug data, so you can see the box2d objects.
Next, 'attach' one of your sprites to the box2d object - i.e. set the sprite's position and rotation to that of the box2d object (corrected for physics world vs screen coordinates)
Once you have that, you can start making more complex Box2d objects, and attaching more sprites to them.,