I have a April build/July delivery on a 991 GT3. The OEM spec is a Michelin Pilot Sport Cup 2 305/30zr20 tire on a 20x12 wheel. The 12 inch width of the wheel seems better designed for the slightly wider Michelin Pilot Sport Cup 2 325/30zr20 tire. While it might look better on the car, it would increase the rolling diameter by 1/2". I'd like to know whether the 1/2" increase in rolling diameter is enough to throw off speedo, tach, etc., and cause problems?

 

Doubt it'll cause a major issue but it will make the car understeer more, so I wouldn't recommend if for track use. If you are looking at it aestheticaslly only, then you are unlikely to throw anything off. Another option would be to consider a 325/25 tire

 

Doesn't changing the overall diameter affect the stability control system adversely?

 

I should have edited my post. If he uses a 325/25 tire, the diameter will be very close to a 305/30. I did that on the C4S and had no issues, and that is a more sensitive car due to the allwheel drive. You are right, 1/2 inch may be an issue so I stay corrected.

 

305/30/20 is 691mm diameter.

325/30/20 is 703mm diameter.

325/25/20 is 670.5mm diameter.

I don't see how the 325/25 profile is better.

 

How would changing the diameter (or more importantly, the correct ratio of front:rear tire diameter) affect the rear wheel steering? I imagine no one knows. And I'm not sure who wants to be the first to find out!

 

would 325's hit the inner wheel well w/ the RWS fully turned???

 

I agree RWS an important factor and for track use, I wouldn't change a thing in the rear. I even resisted rear toe adjustment my mechanic suggested. For street use and looks, yeah why not, especially if there is clearance.

 

also would the greater contact patches put strain on the RWS ???

 

I am getting delivery of my 2018 Manual GT3 in May, i will but set of 325/30 and 325/25, I will try both, on the street and on the track, I'll tell you which ones work the best. My daily driver is a 997.2 Manual GTS and I put 2008 GT2 tires on it (rear 325), they work perfectly and look awesome! If the OEM 20" don't work, I am taking the car down to 19" which i personally like much better as you get more sidewall overall. I am doing all that the same week i get the car. I already have the tires waiting for the car. 19" work but front caliper are mega close to the wheel, I already tried before on a 991 GT3 which I hated it because of the PDK and sold about two months after I got it.

 

Sorry for waking this from the dead but I'm also in the same boat as speedtalent. I currently daily a 997.2 GTS with 325s in the rear and I love it. I'm curious if you never tried the 325/30/20s or 325/25/20s on your GT3. There's another member on the board who's tried 265's in the front with RS offsets without any rubbing so this would complement it well. Thanks