I want to get 305/35/18 nitto 555Rs on the back of my Z with 18x9.5 in the back and 17x9.5 in the front, will it rub on the fenderwell? Also, what size of a tire would i go with in the front?

 

Whether they will rub depends on the offset of the wheel.... how deep it sits in the wheel well. What's the offset (or backspace, if you have that) on the 18x9.5" wheels? Nitto recommends a minimum 10.0" width for those tires.

Front you could use 255/45-17 or 275/40-17.

 

I have Nitto 555 275/40/18 on 18/9.5 Smoothie 2 rims on the back of my 98 Z28. There is a little room left and I have probably 1/2 in I could go in either direction and still be inside the fender well. I have 245/45/17 with 17/8.5 rims on the front. The front is fine, but if you want bigger tires on the rear the rim would have to be offset to the outside some. I can't remember what the offset is on mine, but I will try to find out.