Since my Sugru wheels instructable had a big success, I understood that making good wheels for robots is a common topic.
To improve the grip of the tires I tried to create them with liquid rubber. Although you need a good practice to mix the components and obtain a good result (and you'll probably find better tutorials about liquid rubber use), this instructable can give you an example to create very good tires (and wheels) with no specific molds.

Step 1: The Rims

As I usually do, I will start from waste products to make my robot wheels. The blue caps come from many milk cartons, but you can surely use any nice plastic cap from any package. The more the grip is embossed the better your tires will come out.
The blue caps will be the wheels, and now we need something to keep the liquid rubber inside to make the tires. So search some caps or circular containers (little plastic cups, or some plastic package, also a cardboard pipe could be right) a bit wider than the wheels. I suggest this container has a flat interior, with no thread, but you can try also a threaded one to see the result.
The brown caps are secure caps for medicines so I could take off the inside piece, as you can see in the second picture, to have a clear surface on the inner side of the external cylinder.

Step 2: The Match

Now simply glue the smaller cap inside the bigger one, use any glue, but pay attention to not let the glue go to fill the bottom of the cavity you'll create. The more this "mold" is clear, the better your tire will come out. If you use a vinyl glue, or any not-instant one, you can re-position the inner cap on the exact center of the external one, this is extremely important.

Step 3: The Blend

This is the first package of liquid rubber I bought, and I admit it's not simple at all make a good product, you'll probably must pay attention to:

mix for some minutes both components in their bottles before use them
don't create air bubbles neither mixing or combining them
weigh the exact proportion of components with an high precision scale
mix the mixture for the right time right after combining the components
don't wait too much to pour the rubber into the mold after mixing it

anyway ask someone skilled for some advice.

Step 4: The Pouring

Pour the mixture in the molds, let them react for the right time on an exact horizontal plane, the push hard the diameter of the caps some times to extract the tire. Since the caps are a bit different you can see different geometric patterns on the rubber tires.
Cut the imperfections and examine your new tires. Mine are very far from being perfect...

Step 5: The Tire

Now you can decide whether to keep the tire with the external flat surface, or flip it over to reveal the grip, as you see in this first picture. The grip of this rubber is phenomenal, I think it could be great for little sumo-robots!

Step 6: On the Robot

If you glue the milk caps on a motor (here you can see two very cheap stepper motors) you'll have a perfect transmission system for your robot!
You can see in these pictures the tire mounted regular or reversed (last photo). See you in the next sumo-robot challenge ;-)