A little late, but here's what I did. First of all realize that tires are not round when viewed from anything other than straight on, it's a little more complex shape.
1) Use the pen tool to select 2 points on the outside of the tire. Then use the curve function for drawing curves with the pen tool. Do that all the way around the tire. Then convert to "Selection".
2) Select "Smooth" from the Select drop down list. Increase smoothness enough that the circle becomes smaller than the outside of the tire.
3) Select "Grow/Shrink" from the Select drop down list and increase circle size to size of tire.
That should give you a relatively smooth and round selection.
You will probably have to erase some pixels or clone some on the outside/inside of the tire.
1) Once you're selection has the marching ants it's easy to clone inside the circle with the clone tool.
2) If you have pixels outside the selection, just Invert selection and use the erase tool, but only erase the extra tire material and not the rest of the image.
One last thing... I've added more black tire to the tire before I made the selection as stated above. It doesn't have to be neat or round. After I made my selection I just deleted the extra black outside the selection.