extruding circular shape How do you extrude a circular plane in two dimensions (e.g. z and y)?

I have a wheel with a tire. I have the rough animation of the wheel, but I cannot figure out how to extrude the tire in two dimensions. I've tried with extrude and scale combined but gives me the result seen in the picture.
To scale in one plane, use Shift+ the axis you don't want to scale in.

For example, if you want to extrude and scale in the YZ plane, you would do E&rarr;S&rarr;Shift+X.

If you use the 3D Manipulator, Shift+clicking the handle of the axis you don't want to scale (e.g. Shift+click the X-axis handle for the YZ plane).