Tires for 18x9's on a Mustang for autocross

I'm hooked on autocross with my '04 GT. Right now it suffers from the previous owner going with a staggered setup (18x9 & 18x10.5) with 265/285 sumitomo HTRZII's.

I want to square it up and have 18x9 rims already. I'm shopping for Direzza ZII's and am stuck choosing a size. Off the bat I wanted 275/35R18's (I know they wont rub) which have a measured rim width of 9.5 and a minimum rim of 9. But the more I look into it, the more "rules of thumb" I find. The common one is rim width within a certain range +/- of tread width, some say +/-.5", some say 1".

To further complicate things, Direzzas seem to run wide when it comes to tread width despite similar section widths and same measured rim. The 265/35R18 sumi's on the front of my car now, look like they can stand a wider tread width. The Sumi's have a 9.6 tread width which put them almost 1/2" away, while the same size Direzza has a 10" tread width a full 1" away.

Right now I am leaning toward a 265/35R18. Maybe I am overthinking this.

Any help would be appreciated.

Can you run 10.5s in the front? You may want to go that way instead on such a heavy car.

For a low-profile tire on a race car, you should stick to within 0.5" of the recommended rim width to minimize the chance of debeading. People get away with more and tell me I'm too conservative about this, but that's my advice.

My Z1SS are definitely a bit wider than usual for their size, but look at section widths for tire-to-wheel fitment, not tread widths, If the Z2's specs match up, go for it.

Welcome, I recognize the username from LMMC-CT

I agree with going with the square 18x9 setup, and it sounds like the 275s are a good option.

Rivals or RS3s are other good options if you can live with their wet weather handling (or lack thereof).

I can't fit a 10.5 in the front without a spacer which will move the tire out of the fender.

Recommended rim width for the 275 is 9-11", for the 265 its 9-10.5 and both have a 9.5 measured rim width. So with a 9 wide rim, I am just at the small limit with both options. I'm not worried about safety, I'm worried about having a wider, more expensive tire not perform as well as a more narrow one.

I did notice that 03/04 cobras came from the factory with 17x9's with a 275 tire as OE.

DJ, any idea what happened to the LMMC site?

I would search on Corner-Carvers or on Vorshlag's own forum, Terry Fair has posted a ton of measurements and tire & wheel fitment info for later mustangs.

For reference, the 205 Direzza Z1 which spec a 8.07 section width are very happy on an 8" wheel on my Miata, and the 225 with ~8.9 section are happy on a 9" wheel (per miata.net).

I ran those Kumho Ecsta XS tires at autocross, they are inexpensive and good.

Direzzas seem to be old hat now not many people use them where I am anymore.

In reply to kanaric:

The Direzza ZII's are cheaper than the Kumho XS.

Z2s are also a better tire overall from what I've seen. The Kumho XS can product more grip at the limit in the dry, but that's the only thing it has going for it.

It took me long enough to realize a good reference might be to see what the same year Saleen S281's came with which is silly because I did buy Saleen replica's. They wore 255/35 front and 265/35 rear with 18x9's at all 4 corners standard. Optional tires were 265/35 on the front and a 295/35 on a 10.5 in the rear.

265's make even more sense.

Kumho XSes are only happy on very wide wheels--they have very soft sidewalls.

Get the ZII (the new Star Spec version is here) for such skinny wheels, but I'd still recommend sticking with a 245 instead of a 265 or 275. Stick the 245 on a 10.5" wheel first.

Among all the ST* tires, the Hankook RS3 seems to be very popular among owners of heavy cars.