On my 14 I have an 11" rear wheel with 315/35-20's on it.
I know @ least one site member running 325's..

 

It all depends on the wheel specs. The link below is a good read!

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2007 - 2014 - Wheel/tire fitment guide

I have a good bit of practical experience with wheel and tire fitment in my 2012 Shelby. While a few refer to me as the site expert, it’s only because I have worked with several different wheel configurations and tire sizes than most have done. Let’s face it, a mistake buying wheels and tires...

www.fordgt500.com

 

Here is a 2013/14 owner with a 325 rear 275 front combination on widened Alcoas.........






I have the same Alcoa set-up on my 2008........




I did mine many years before the GB 2013/14 owner, he may have read my forum posts.....


R

 

These are 11.25" with a 7/8ths spacer to keep center line of wheel identical to stock.

 

325/30-20 on 20"x11.50" for me. That combination is 13.5" at the cross section. You can go wider, but the tire will be outside the wheel well. Offset is critical.

 

Wow! that is a lot of tire under the car. I believe I am at about 12.9 cross section. I still get a little rub when cross cambering in and out of driveways. I cannot go any further out without sticking out the fender an objectionable amount.

 

The Alcoa's are especially tricky because the actual face of the wheel is so far out at the fender edge. It takes some careful tire fitting to only have a small amount of tire bulge out past the fender while keeping all tread surface under the fender. With a straight edge across my rear 325's, I have 3/16" tire bulge sticking out past the fender lip, any more "bulge" on the tire sidewall would stick out even more.........<<<This would also have been the case if I would have chosen to go narrower on the Alcoa widening and still keep my 325's, the bulge would have been greater and well past the fender lip. 3/16" for "tire bulge only" past the fender lip seems to be an acceptable look, in fact it is not really noticed unless a person stands over the rear fender and looks down across the tire.

R

 

325/30/20 on the rear on mine with Carroll Shelby CS14 Wheels. Sorry not a very good picture.

 

Great shot to show the fitment of larger rear wheels/tires. If I read correctly, you have CS14 wheels. These should be 11" with 50mm offset. I think that this should be putting the wheel .75" out and .75" in from the stock 9.5.

 

Maybe there are different specs? I looked at website prior to post to check on the specs. This is the info that I saw listed. Not doubting what you are finding. Just seeing this listed for the 11" CS14 wheel. Could be more than one part number of CS14 11" wheel available for all I know. Either way, it should be a subtle difference. Backspacing between the two are 7.97 as seen here or 8.125 as you have attested to having found.

SPECS

Backspacing (in)	7.97
Brand	Carroll Shelby Wheels
Center Bore Diameter (in)	2.783
Center Bore Diameter (mm)	70.7
Center Cap Included	Yes
Construction	Cast
Diameter	20
Diameter (in)	20
Emission Code	6
Material	Cast Aluminum
Offset (mm)	50
Product Type	Cast Wheel
Quantity	1
Weight (LBS)	38
Wheel Bolt Pattern (in)	5x4.5
Wheel Bolt Pattern (mm)	5x114.3
Wheel Construction	Cast
Wheel Series	CS14
Width (in)	11
UPC	842110184498
Part Number	CS14-215455-G