We've been using a 6 inch
Skyway pneumatic wheel(listed on this page as '5" caster wheel") that we've had lying around from a robot many many years ago. It worked great, but we ordered more, and the tire has a tread that is different from the one shown in the picture, which is also the one that we had. Another issue is that the wheels seemed to wobble when running them. This was all after today, and we've yet to find another 6 inch pneumatic wheel (Smaller sized pneumatic wheels have been hard to find...I searched A LOT for one last year...)