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On track, having a 255 on a 10" width wheel is only a couple tenths faster on a consistent basis. How the S2k Challenge base their point system is through the testing on Buttonwillow Raceway. There is sufficient data to support the 255 on 10" vs the 255 on 9". The difference is approximately 0.25 seconds on a 2.68 mile track, hence the 0.25 point penalty. The key is consistency at this point. Also, I'd like to point out the S2k guys run either NT01s (R-comps) or Hankook RS3s/Dunlop Z1SS. Everything else is too expensive and unproven to go faster that those setups thus far.
On a side note, autocross guys stuff a lot of tire because in the end, grip is grip, so more tire helps give you as much possible grip you can get on your car for the autocross course. The speeds are not fast enough to cause you to slow down. On large tracks, too much tire on a particular car can actually scrub off speed in your corner, reduce the speed on the straights, etc. This is why some guys running 255s on the BRZ/FRS are not running any faster than guys with the 215 size tires. Now if the car was turbo'd, then 255s are the faster way to go...:p
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