R Mackow

I have std. C5 wheels; 8.5 x17 and 9.5x18. I have 275/40-17 front and am about to buy 285/30-18's for the rear. This keeps me in NCCC I-H, because I am running stock wheels, but can change tire sizes.
Question: I there any disadvantage to using a sightly larger diameter tire up front. Tire rack lists the diameter as 25.4" for the 275 and 24.6" for the 285.

Jason

I can't think of any advantage to a bigger diameter tire in the front.
Just more likely to rub the fender liner.

FastZR1

Are you sure you can change tire size and still stay in GR I-H? I seem to remember (without looking in the NCCC rulebook) that you couldn't change wheel/tire sizes for group I.
One exception is in I-G where a standard C4 could run 315/17x11s out back since the GS came with those sizes. But for the standard C5, you only had the one tire size up front, right?

TedDBere

The only issue you'll have is your computer will have to be turned off. It'll think your rear tires are spinning and activate traction control. Also you rake will be negative. I ran that combination for two years with Hoosiers BTW.

Bill Dearborn

NCCC Group I Rule 2.7.2.19.2 on page 2-19 says the following:
Tire width and profile must fit the correct wheel and size for class and axle.

I would read that as the if the tire manufacturer's spec's say it can fit your wheels it is OK to run that size (be ready to prove it). If the manufacturer says it doesn't fit then you will be put in Group II.

Bill

69autoXr

Any size tire that will fit the stock wheels is for Group 1.

Gordy M

The 275's up front will not rub so there is no problem. The other problem that TedDBere mentioned will have to be addressed. That is why I went with the Hoosiers, a 295/30/18 rear is the same height as the 275's and you do not have computer problems.

I found this set up to be moderately successful in SS (SCCA) competition and very successful in IH (NCCC).

R Mackow

Thanks guys. About the computer that is. I normally rule w/o traction control, but sometimes do competitive mode.

Regarding the legality of staying in I-H..I'm OK. I've had the regional governer check last year about wheels and tires and the confusing NCCC language. To simplify the NCCC rule: Any tire that can be put on a stock C5 wheel is OK and by putting the 275 on the front, obviously I cannot compete in I-SH, can in I-H.

I understnd the rule book is being revamped to be more readable.

R Mackow

Hey, the 275's do rub a little if you crank the wheel all the way. Otherwise there're OK.

mountainbiker2

My buddy put on 315/35/18 V710's on stock rear rims all the time. 9.5" Those 285's have to small of a sidewall for a rear tire.

Steve

TedDBere

The only non-Z06 C5 to win the National SCCA SS title, ever, was on 285 rears. They're just quicker because they're shorter.