hi guys

my 12' g37xs have 225 45 r19 wheels when it came out from the factory, im planning to buy 265 35 r18 with nankang ar1 tires for track use
since the tire size become much smaller, would that be any issues for the trans or diff? or get 245 40 r18 to make it less difference on the size?

 

What wheels are you planning to run, keep in mind w/ your big S brakes you'll need to confirm wheel clearance. Since you run AWD I'd try to stick closer to OEM diameter, there's not a lot of real world data as far as how short you can run w/out causing problems. If you run the same tire size all around I'd try to stay within 3% of your OEM size.
Running a 265/18 all around you'd want a 9" to 9.5" wide wheel w/ an offset in the mid 30s for best fit. For a 245/40 8.5width is good and you could run offset in the 40s.

A 265/40 would be a good choice if you can find a tire in that size