I just started autocrossing my gtp and I was wondering on suspension upgrade suggestions and also What is the largest and best tire and rim fit for my car (road coarse set-up). Suggestions on brands?

For one the GM handling improvement kit available from GMpartsdirect.com(service is suspect)but you can get the part numbers and order the parts through your local stealership. As for tire and wheels I'm no help there. And off the top of my head I don't know of another member who autocrosses here. They're all into the drag strip. Hope you can get this beast of a car to handle well and get some wins.

I think Brian autocross' a Bonneville.
Yes, the handling upgrade kit would be good, along with a set of GMPP strut/spring combo.

I just finished a school in St. Louis area and I was running 45 second laps. The instructors were running 40 seconds in their extreme WRX Sti's. I was beating guys running VWs, BMW's, and other miata's. I was one of 3 that have never raced before. My instructors gave me full props and were surprised how I could move "that boat".

Hello,

The widest tread I've heard workable on our cars is 245cm. You can go any size rim you want, the aspect you need to pay attention to is the offset, or backspacing....Pending on who you talk to.
If you are going to compete, I would suggest the Koni/Eibach combination. The GMPP setup is said to be the same thing, but those that have gone with the GM setup would tell you different.
If I were to go with an upgrade, I would take the GM setup, but I'm not competing.
Coil Overs are another option. Not sure where to get them, but we have a member with the setup, and they are extremely firm at any height. He purchased the car with them, so he knows not where to find them either.

Later now,
Clint