If I grab 18" XXR 962's with Tires: 245/40/18, 275/35/18. will this produce the same stock gap as with stock tires and rims. I own a 06 350z all stock. I may just go 19s if it doesnt

 

How tall is a 245/40/18, compared to the stock 225/45/18?
How tall is a 275/35/18, compared to the stock 245/45/18?

Your front tire is taller than your rear tire.

 

275/40 is the proper tire, and then the gap will be the same. If you get a more aggresive wheel than stock, it will give the illusion of the gap being smaller since the tire will be closer to the edge of the fender.