Recently got a 2012 G37xS coupe, and the tires are in desperate need of replacement. After browsing on here I've seen a lot of people running 245's up front on RWD models, and some talking about 245 all around on awd models.
For a number of reasons (especially adding some curb protection to the rims), I was thinking of going wider than stock. From checking some recommended calculators, I had come to the conclusion that a 255/40/19 would be closest to the stock diameter, but the guy at the costco tire center is unsure about if his manager will approve putting those on as they're at the upper limit for width on the 8.5" width stock sport rims.
He suggested a better fit would be 245/40/19, which are not quite as close to stock diameter, but they're within 0.9% which I am led to believe is good.

All that being said, I'm curious if anyone can describe what the handling differences are between the stock 225 width a 245 up front. I guess in theory it may be harder to turn and more grippy, but I'm wondering how big that affect really is. If it's going to make the handling too "heavy" for my girlfriend to drive comfortably on the pothole filled city roads, I may just not bother and go with stock width.

 

I don't have the right vernacular to talk about handling differences. That being said 255 are perfectly fine on a 8.5" wheel. Yes they are at the "upper limit" of acceptable mounting, but they are within the limit. Hell I am running 275's on a 9" wheel on my car (not a G but a 3000GT) without issue. On the note of 3000GT the TT's come from the factory with 8.5" wheels and 245 tires. Plenty of guys run 265's on stock wheels so 255's won't even be close to being an issue.