What tires for autocross?
 

Stock GT tires are 225/45-18s. TireRack doesn't list any competition tires in that size, but do list three (Kumho Ecsta V700, Michelin Pilot Sport Cup and Pirelli P-Zero Corsa) in 225/40-18.

Going with a 40 series tire would effectively increase the final drive ratio and thus provide slightly more torque. Any thoughts on going this route? And does anyone know if that would still be legal for SCCA stock class?
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Make a vaporizer

Re: What tires for autocross?
 

Should work fine, and yes, you can fit any size tire you like on the standard width wheel in Stock.

By the sounds of it, you're looking to run these tires on the road as well as in competition, right? If you're looking for a strictly competition tire, you'd be better off with Hoosier A3S03's in 245/35-18, but at $1000 a set you'd better hope you don't flatspot 'em too quickly. :)

In general, is it better to go with 245-35/18's on 18x8" wheels or 225-50/16's on 16x7.5" wheels for autocross?

That brings up another question, which is which class could the RX8 be the most competitive in? It seems like the choices are BS, STX or SM. For some strange reason BSP has a worse PAX modifier than SM does.

STX would require the 7.5" wide wheels and eliminate race slicks. I've heard good things about Falken Azenis but they don't make tires in that size. 225-45/17's are the closest which should fit on the RX8.

I've been autocrossing for less than two years now and I'm a little new to the whole minmax design strategy and car classification thing. Any suggestions would be very welcome. Should I even mention the idea of going into the Street Tire class?
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Fisting German

The Michelin Pilot Sport Cup tires are outstanding track tires with great grip and longevity. However, I don't think they'd be very good for autocrossing, as they take several laps on track to warm up.

STX allows a maximum wheel width of 8" and a maximum tire section of 245.

This really isn't relevant at the moment since SCCA classes the 8 in BS. This could change, but not quickly I think. Mazda may have an issue to deal with here since BS is "owned" by the S2000. Mazda cars rule some other classes, particularly CS with the Miata. But this is only autox. If you look at track classes, Mazda Miatas and RX-7s rule more than one and no where is an S2000 found. Given it's history and marketing stance, "zoom-zoom" and all that (every car has the soul of a sports car), Mazda may have to get Ford in line and let them do what they do best. When I see all the stuff about the 6 (the platform for the new Ford Futura!), the REVITUP program, Mazda ownership of Laguna Seca, etc., I can't help but smell Ford influence over this car. After all, if it's too good, the next Mustang may suffer!!!