Looking to autox I-Roc 84 Camaro what's the biggest size can run or what's the best size to run? One that won't rub too much while driving around town. Thinking 275/40 17"

That's the size I've got and they rub in a couple of places but just a bit. 255 probably wouldn't rub but those are the tires I've got. You could go a little smaller on the front but then you can't swap back to front. It's not just about the size it's about the compound and if you want to run in a particular class if it's an SCCA type event. Plus how much you want to spend. But if you haven't auto-crossed (or any other racing) before you don't need to buy bigger or better tires until you are better than the car.
Several folks in my club really like the Dunlop Digi-tyre Star spec. They do a 265/70/17 which has great grip for not a huge outlay. Have fun whatever you do.

that sounds like a truck tire.....?

Thanks Base some good advise I will probably take you up on that. 265/70/17 should be a good start. Yeah trying to get in SCCA type stuff.

thats like a 32" tall tire. lol

Base I'm assuming means 40 not 70 because that would be more for a truck

Yes. It was late.

You can easily fit a 17" x 9.5" wheel all around with a 275-40-17 tire and not have any problem - in fact my company makes wheels for that application that require no spacers.

Definitely want to run the 275 all around as the cars are front heavy and need a lot of tire up front. You can use a 265 like someone else mentioned as they are minimally narrower - and a really good tire too.

If your class allows 17" wheels (I know CMC does), you will be at a marked disadvantage in grip running a 16" stock width wheel. For learning purposes and budget, the 16" is a winner...