I am asking specifically for the Falken Azenis RT-615s on SSR Comp-H 18x8 wheels, as the prices are comparable for either tire size (235s slightly cheaper). I will be getting these new tires for this upcoming autocross season (unless convinced to another tire), and will probably be doing a track day or two this year as well.

My understanding is that the tradeoff would be in responsiveness vs. contact patch; with more responsiveness in the 235s with less sidewall flex, and more contact patch with the 245s. For autocross application, which should be a priority? Would differences between the two sizes be too minuscule to notice?

EDIT: FWIW, these tires will also be my daily driven shoes.

245/40

I don't think you will notice much of a difference between the two...But most people who AutoX always go with the maximum tire size for B Stock, 245's.

Many of us have used the 245/40's. It seems to be more close in diameter to the OEM tire size, so there isn't a change in the odometer/speedometer. The 235/40 would give you a little more acceleration as it's a little smaller in diameter.

275/35! Woot!

Sorry, I just love ultra-fat tires.. I never lose traction ;)