The_Backmarker

Quick question: Will a tire in the 275/35-18 size fit OK on an 18x10.5 C5Z Speedline wheel?

These would be used for the front tires in a C5Z.

fatbillybob

That's the wrong direction. Stock is 295 on that wheel. Often 315s are run square with 10.5 z06 wheels up front. 275s are better on 10" wheel and with rain tires to reduce aquaplane. There is no good reason to go narrower unless class rules demand it.

The_Backmarker

Yes, this is a class rules thing (Global Time Attack - Enthusiast RWD class). Max tire width is 285mm. I think what I'll do is find some base C5 rear wheels (18x9.5) and use those for the front wheels with either 265 or 275 tires. Alternatively, I might be able to run 285/30-18 on the 18x10.5 Speedlines up front, assuming no clearance issues with the front fenders.

Kubs

Its a perfect fit. That combo was king back when NASA's TTA class was around. I still run a 275/35R18 on 18x10.5 wheels.

This picture shows a stack of 275/35R18 on 18x9.5 wheels vs that same size on 18x10.5 wheels.


Here are the 18x10.5 on the car:

93Polo

If you go that route some of the thin spokes had stress problems under track use. You could find some C5 wagon wheels inexpensively.

sperkins

Exactly. I still run them in NASA TT3/ST3. It's a fast setup.

mattastick

Are you on 200tw as well? 285-30 Bridgestone is going to give you awesome response on the front, but was limited in terms of multi-tasking ability, from my experience (all autocross, but on 11" wheels, rather than 10.5s). I switched to 275/35 Rival S' and have had much better success. I didn't have any issue with clearance (just try to limit yourself in the paddock to 1 full turn of lock). You'll have a little more than that (sit stationary and turn the wheel, and you'll be able to feel where you start to have tire to control arm contact), but it's easiest to just tell people driving my car 1 turn of lock each direction from center.

The_Backmarker

Thanks for the feedback, guys. GTA's Enthusiast Class requires a tire with a TW > 220 so that limits tire choices. My plan right now is to run Continental ExtremeContact Sports. I've had good luck with them before.

Another question: Will I need to run a spacer up front with the 18x10.5 wheels?

Scooter70

No spacer needed. C5Z rear wheels on the front of C5s/C6s is a very common usage.

Rx7Rob

This is good info!

I'm also changing to 10.5" Z06 wheels on all four corners. I'm not thrilled with the suspension height change required to get back to stock rake when going square with 315-30-18's on all four wheels.

I've been considering 275-35-18 on the front and 305-35-18 on the rear. This will be almost the exact same tire diameters as stock for my car (C5 non-Z). So rake will be correct without screwing up the lower control arm angles. I was concerned the short sidewall on the 275's would result in increased understeer (will act like a stiffer spring). Sounds like thats not an issue!

SocalC5Z

I've been running 275/35-18 square on C5Z 10.5X18's since I decided to try the Maxxis RC1 about a year ago and there are zero issues. In fact the measured width is nearly the same as the 285's I was running before. Most of the R comps run wide to maximize rule limits. The car is fast and balanced. As much as I'd like more rubber and a larger contact patch, I'm not comfortable with the compromises of smashed front brake ducts, fender/tire contact, and loss of grunt from a taller tire. YMMV

RC1's on 10"X18"
.

Mrsideways

I ran that size on the good years at road atlanta this weekend. It worked fine.

mattastick

The Pilot Sport 4S is going to be faster than the Conti Sport, but significantly more expensive. So if outright speed is the goal, go with the Michelin.

JHrinsin

From what I have heard and read, several folks claim that the slightly older Michelin Pilot Super Sports tend to hold up better to track abuse compared to the new Pilot Sport 4S. As always YMMV.