if your OG rims are the g37 18s and your moving to the g35 19s the best option for the would be 245/40/19.
reason being:
wider tires result in an increase in overall diameter in the wheel+tire combo. consequently, your circumference of travel is greater! a side effect of this is that your speedometer will be off. i.e., your speedo would think that you're traveling 60mph with the old setup but in actuality, with the wider setup, you're going 58mph; it is slightly less because you're only traveling a portion of the larger circumference.
the easiest way to fix this is by trying to match the diameter of the original setup to the new setup to accommodate for speedo accuracy. if you dgaf about this (like most people) then you can run pretty much whatever you want.
original setup:
the stock g37 18s come with dunlop sportmaxx a1-a/as 225/50/18 rated at 776 rev per mile
(225*.5)*2+18*25.4 = 682.2mm
sectionwidth(225) * aspect ratio(.5) = section height (multiply this by 2 for both sidewalls). now, add that to the rim diameter (converted to mm) to get the total diameter.
prospective setup:
(245*.4)*2+19*25.4 = 678.6mm
this is pretty much the closest you can get in terms of diameter with any setup.
now, of course the physics guys will say that these are purely ideal numbers! in fact, when mounted and supporting the weight of the car, the diameter is supposed to decrease by 3% and is forever changing and really unknown do to your weight, stuff in the car, hard turns, etc... the most accurate way to account for a speedo change is to match the REVS PER MILE of the prospective tire to the original tire as best you can. keep in mind, that the rev per mile will differ among different tire manufacturers even if both tires are the same spec because of the quality, design, etc... the re11's rev per mile for 245/50/19 is 779 so that's probably as best as you can get. you can go 285/35/19 but that's too much pinch for a 8.5 tire.
you should do the same calculations if your g37s came with the 19s or just find the rev per mile of the potenzas it came with. the reason for this is that the speedo is based of the original setup and would be (and i'm purely hypothesizing) adjusted according to the regular18s or sport19s
calculations:
http://www.tirerack.com/tires/tirete...e.jsp?techid=7
best tire calculator that accommodates for the 3%:
http://www.1010tires.com/tiresizecalculator.asp
another way to do this:
http://www.tirerack.com/tires/tirete....jsp?techid=36
good luck!