Somebody earlier this month asked the same thing. Do a search on the forum for cars or wheels and you should find some useful info. What pauljs said is essentially correct. You can right click on your scene and a list will pop-up of all objects under the cursor, then you can select the tires without having to ungroup. When animating the rotation try doing with auto-key off, otherwise you'll get unwanted keyframes that will hinder animating the rotation. The auto-key is in the lower left corner on the animation pallette there's a little triangle.