I'm not going to say I know everything about the science of tires, since I don't, but this is what I've gathered from reading around. Use the widest tire the car is capable of heating up. That way, you'll have the most available grip and be able to carry maximum speed through the corners. Of course, a 318ti won't be able to heat up a 275 tire, so that will slow you down. Our cars have enough weight to heat up 225's, and possibly 245's since I've seen some racers use them.
Isn't the compact cup a spec racing class? If it wasn't spec, I think some guys would be running wider tires to gain an advantage over the others. __________________ 
2000 M5 -> 1996 318ti -> 1997 328i 
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