I am looking to buy some z06 dish rims that are 18's and the guy swears they are 10.5's or 11's but he says they have 275x35x18's on them. Is that even possible? Also, he says he has 245x40x17's up front and they "should be" a 9 or 9.5" rim. Does thsi seem accurate? How can I tell?

 

check the back on the spokes, it will tell with, diameter and offset. also, check how the 275/35/18 fits the wheel..if the sidewalls are flush with the rim, most likely a 9.5...if they look like they stretch, then the wheel is wider

 

They are actually deep dished rims and he said they actually bulge out a bit. I don't understand how that could be, but oh well.

 

Its is the tire manufacturer that decides what width wheels his tires are designed for. Most (but not all) will accept a wheel width of 9" - 11" for a 275/35-18 tire. Kumho is an exception, for example, recommending 9" - 10.5" for that size.

And, most will recommend a wheel width of 8" - 9.5" for a 245/40-17. You wouldn't want that size tire on a 4th Gen, though, because it is 1" smaller in diameter than the stock tire.

Because he apparently went to the upper limit on wheel width for both tires, they are going to look a bit "stretched" and will project so little beyond the edge of the rim that they will offer nothing in the way of curb protection.

A factory C5 Z06 rear wheel is 18x10.5". If you are getting some sort of custom "deep dish" wheels, who knows what the size or offset is.