The answer is you can use 315/30/19 or 325/30/19 on your Sig wheels for the rears. You can use 305/30/19 on the fronts as well. I had the bolded tire setup on my Sig wheels. A 35 sidewall will ride a little more comfortable than a 30 but I found the 30 sidewall PS4 to pretty compliant. But I was coming form an R SC2 30 sidewall. I don't even see where the 35 sidewall is even offered for 315 or 325 in PS4.
There's a gazzilion guys on here running 325/30's on the back. They are perfect. You also have the perfect size wheel to run those. Your focus on the R vs non R is not as much a factor as you seem to think it is either. Darn near irrelevant between the two cars when it comes to tire size and fitment. The cars are identical. But to ease your mind, here's 325/30's on Signature R spec wheels (same as yours) on a non-R car.
On a personal note, if you're stuck on the 4S, I (and just me personally) highly suggest the 315 over the 325. It has nothing to do with the fitment or functionality, nor performance. It's aesthetics based. The Michelin's run huge (not all brands are the same size in the same size, if you get my meaning) and I straight up don't like the looks of the 325 Michelin.
But doesn't the GT350R use the smaller diameter tires anyways so why would the stability control be out of whack?TireRack is gettting 305/35/19 SC2s are in stock and they are getting the PSS on 22 Oct. On my 2020 GT I run 305/35/19 in the rear and I bought a set of used PSS with 80%of the tread still on them as I couldn't find a new set anywhere. I won't run the Nitto G2s, garbage tire.
To your question though, you can run them. You will just have more fender gap and your stability control may be out of spec due to the difference (quite large)of the diameter of the tires.
Thank you. If it fits in the stock GT350 wheels, I m guessing the 25/55mm R offsets I have will be plenty as they stick out even more. I might still go with 315s because I am worried of the alignment wearing the insides.
Yes but probably adjusted for with the as built file.