What is the widest tire I can run on a stock 17" x 8" GT rim for the rear of my 2002 V6 Vert? 275/40?

 

The widest manufacturer approved tire for an 8" wheel is 245's. You can squeeze more than that, but would not go past a 255. Hell, a 275 is really too wide for a 9" wheel, in most cases.

Jazzer

 

It really depends on the type of tire. Usually that is the case but Nitto 555R drag radials run narrow and many have put the 275/40/17 on a 17X8 stock bullitt rim with only little buldge.

I have that exact same tire on my 17x9 stock style rim and it runs pretty narrow with little to no bulge: