Rocket, Do you like the look of the 315's you have and are they the Michelin PS 2's? Do they look any shorter than your stock Z tires? The PS2's are around a half inch shorter than the stock Z tires.RocketSled said:345s won't fit in the rear without a Tub. 315s will. Depending on who you ask, 335s might or might not hit the lip of the wheel well.
315s are also rumored to fit up front, but I have 285s and they're tight.
You need to keep the front/rear diameter ratio within a certain range or you'll screw up TC and AH. 285/30-18s up front and 315/30-18s in back are what I have. They keep the wheel ratio in a "good" range. I'd recommend that.