IRC tires stick a little better but cost a lot more than the TT91s.  YSR racers still use T90s even though they were replaced by the stickier TT91s.  But, you don’t see too many TT90s on the track today.  I noticed racers who use IRCs at Prairie City inflate their tires about 3-6 PSI more than TT91 racers.  I tried it with my TT91s but my bike felt loose and skittish on the track. I usually inflate my tires 18 psi for 70 degree weather to 22 psi for 85+degree weather.  A big plus to note when racing YSRs: whatever tire you finally decide on should last the whole season.

IRC 740 (Joe’s and Alvin’s choice)

DUNLOP TT91 (Craig’s and Eddie’s choice)

DUNLOP TT90

RACE TIRES ARE PHATTER... err I MEAN FATTER

For comparison: STOCK vs DUNLOP TT90

FRONT tires are on the left, the REAR on the right

New tires range from $120 to $160 a pair. All the race tires listed above have the same same tire dimensions. I don't think you can go bigger. The rear is maxed out so the chain is a few millimeters from hitting the tire!