Around here for rubber 24mm Sorex 26r's with HPI green inserts are what I use. More people are starting to use foam tires as well. It all depends exactly where you are going to be racing and which car you have. You might try posting in some of the racing threads for the local area. Try the O.C. Circut and RC Heat threads for parking lot. There is also Revelation Raceway, Marvin's and Chrystal Park for permanent tracks. There's racing almost every weekend.