Thanks guys, I went with 245 just to play it safe. My GTO sags a lot in the rear as well and I need front tires, could I go with 235/35/18 up front to lower the front a bit?
You should check how many rotations in a mile each tire size makes. I dunno how close the front to rear has to be, but if there's too much difference, you'll have ABS and traction control problems and maybe throw some codes.
I would have gone wider and made them fit. With fender work (easy), an upgraded suspension and the right offset you can get quite a bit wider back there. The reason for the suspension work isn't for ride height tho. It should be able to sag to the bump stops without rubbing if it's done correctly.
Ya you want it to be at a good height but contact with the inner or outer fender is more about how well the rear cradle and control arms are being held by the bushings under load. IMHO inner and outer control arm bushings with subframe bushings hold the wheel in place in relation to the well. There's a decent amount of play under lateral loads on the stock bushing and with the voids in them.
When I put 285s on 17"s on my car I replaced the bushings, jacked the car up and removed the rear springs (pretty easy). Then I mounted my wheels back on and jacked the suspension up and down and noted where I was getting close/touching under the entire travel. A little BFH to the inner fender, 5mm spacers on the wheels w/ARP studs to bring the wheel out a touch and a moderate fender roll allowed the particular wheels/tires that I'm using to fit.
When I had full travel up and down without getting too close to the inner or outer fender I put the springs back in and buttoned it up. I've hard zero rub under every possible condition for the last 3-4 years.
With stock wheels you have +48 offset. Probably most of the work would be to the outside (cutting/rolling) but you could have the wheel hubs shaved a bit too if absolutely necessary.
I went with BFG Sport Comp 2s in 245/40/18 on my rear factory 18s.
They fit perfect with about a finger width between the uncut rear fenders and the sidewall of the tire.
The guy at the tire shop mentioned that the 245/40s were a bit taller than the factory 235 40 18s, so i hope that dosent mess my speedo up at all.
I like how the 245 40 18s fill the wheel well up a bit more than the 235s.
I also aligned my cradle prior to putting these on, just to be sure. They fit fine, but I dont know if I would chance going with 255s. I wanted something that would fit with not fender cutting.
Also, I still have the Bridgestone 235s up front, and I have yet to have a code thrown due to that issue of the tires being different heights.
Of course then there is always the issue that different brands of tires seem to spec out wider or narrower per same size tire than other brands. ( ie Nitto is on the narrow side or so i have heard)
When I have the dough I will do 245s on all 4 corners.
Hope this helps