I make a lot of robots with Parallax continuous rotation servos
and I always wish that there were standard sized wheels that could be zip tied to the servo horns. Well, thanks to the power of 3D printing, I need wish no longer. I designed three different types of wheels for use with these servos in particular. One is a perfectly round wheel. The other two have treads (a wide tread, and a tight tread). I printed the tire on an Objet Connex 500 printer using a 30/70 plastic to rubber blend, and the hub using a 65/35 plastic to rubber blend. I made the hub use a digital material because it tends to be stronger than the pure Vero White plastic resin.
If you are using an extrusion based printer with ABS, I would suggest sticking with the round wheels and putting large rubber bands over the tire for traction.