From 17" OEM tires on rear To 18" any advantage?
 

I am thinking about taking my 17" OEM wheels and tires on the rear and reaplacing them with 18" OEM wheels and mounting like 255 min. tires . My car is an 2004 and I like the looks of the wheels alot, anyone done this to their Z`s? pros and cons. Or just a waste of ?

Performance wise? You are going from a 235 mm tire to a 255 mm tire: an additional 9 percent contact patch. I doubt that you will notice a difference in performance.

I agree, off the line I might be slower cause of the bigger dia. of the wheels and tires but it should be miniscule, I was more or less thinking about looks and maybe a little better on handling around corners.