i have 275-40-18 all the way around on my 04 gt. i have a 1in drop and it only rubs if i have it at full turn. the other thing is with the 40 you will be +1, so your speedo will be off. if you put 35 you wont have to worry.
i ran 275/35/18's for many years....just switched to staggered...255/35/18 front 285/35/18 rear. I just love the look and no rubbing when i turn. And tirerack is very good...i hear p-zero's not so good. I have the goodyear eagle F1's and like them alot...much stickier than the nitto 555's i was running.
I have 265/35/18 in the front and 285/35/18 in the rear with a 2inch drop front and rear and the only way it will rub in the rear is if I have people in the back. The front doesnt rub at all
I have 275/40/18's all around, sportline lowering springs, rub only if full turn, speed cal to correct speedo, so no problems there. Don't like the with in the front, will go 255 next time.