Ive got 18 x9's with 275-40's with a 26mm offset and the back spacing is either 6.11 or 6.5 (i dont remember exectly but i can look it up if it helps) and they looks and fits great

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The wheel sticks out past the fender just a lil, you notice it most in the front of the car but i think it gives it a mean a$$ look. I have seen pics on forums of s197 lowered with the 03/04 Cobra & FR500 wheels on them and they said they have no issues with rubbing. Seems to be a few guys running New Edge Mustang wheels on their S197's, i googled most of my wheels research so theres plenty of info out there.
Also, the lower the offset number, the higher it will stick out. Your 28mm offset wont stick out as far as my 26mm.