I run 275/35/WR18's on my 18X9 and they work very well. Dont stretch the tire like those who try to do 245's just not worth it. I've seen plenty of people run 265's on 9's, but IMO 275 were made for 9's Plus you get 10.7-11" of rubber width on each rim to the ground, quite good for traction
Got 285's on my 18x9's and they fit PERFECT. People are forgetting to mention that it also depends on the manufacturer. Some company's tires run wider than others'.
BTW, i have 245/40/18 up front on the 18x9's and they arent stretche done bit and fit perfect