We use a 8.5"x2" Bead-LOK wheels skyway wheels sells, we then grind them down to about 6-6.5"x2". They have tons of grip, and we have yet lost one due to competition, we had a nasty hub crack in 2006 but kept going on the hub. they sell 5 different sizes ( 8.5x2, 9x2, 9x3, 10x3 and 10.5x3) also your choice of hub type. We have been using these tires since 2002 and have been really happy with them.
http://www.skywaywheels.com/usfirst.htm < FIRST discount
http://www.skywaywheels.com/products_002.htm