TinyRC.com - tires

sandpaper scrapes off crap that gets caked onto the tires. It also roughs up the rubber and exposes some fresher, stickier rubber.

some info on tires:

Tread has to do with stability and lateral grip, and has a little to do with traction.
Hardness affect traction and rolling resistance.

Slicks: When Dealing with official Microsizers tires, slicks are "medium" hardness. That means they have a good balance of traction and top speed. When cornering, they have some lateral grip to help minimize spinning out

Block Tread: Soft compound tires, they have higher rolling resistance than any of the other tires, but they're soft and have excellent lateral grip and good forward traction

Ribbed Tread: Treads going around the tire means less contact with the ground. The hard compound also means there's less rolling resistance. What you end up with is a tire that has some advantages of the slick tires (for instance, no cross treads, so the tire rolls smoothly) but it takes it a step further. Theoretically, the ribbed hard tires will give you a higher top speed than any of the other tires. The other option is that you end up spinning out and using these tires for drifting on desktops.

Other places sell different kinds of tires. RadioShack's Zip Zaps tires are all about the same softness (VERY SOFT AND STICKY!!!) but the tread is different. The zip zaps slicks are probably better then the soft compound block tread on really smooth surfaces where you normally spin out, if you want tires that stick to the road and don't want any oversteer or understeer. They do tend to attract more dirt and grime, and need to be cleaned pretty often, though.