Re: Biggest Tire Size
Whether you're running a Quadra-Lift suspension or not, you may not be able to go any larger than 275/65-18. I've got the QL suspension and just put on a set of Goodyear Duratrac 275/65-18s on my GC. Clearance is pretty close, especially at "Park" height, but I've found no interference at all so far.
I'm only speaking to 18" wheels since that's what I have. I also take my GC off-road, so that's why I picked the Duratracs. Your needs may be totally different, but you didn't specify. If you're looking for something in another wheel size, there are plenty of threads in the forum.