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Sumitomo? My prayers are with you - even though I'm not religious. I was driving one day on an uneven surface and the 928 was all over the place. I was so surprised by the erratic movement that I had to stop and check if one of the wheel was lose. To cut the story short, it was the soft sidewall of the Sumitomo that was wrecking the drivability of the car. Worst tires I have ever used. Now I'm running BF Goodrich G-Force T-A KDW on 17" ~36psi all around. Very happy with that setup...
On two P-cars I've been running 295 30 18 Sumitomo's for over 16 years now and have never had an experience like that. They are excellent tires, and will buy yet another set when they are done. Go take a look at their ratings at Tire Rack which agree with my assessment.
Sounds like to me you may have damaged the tire somehow.
Or just take it to a competent metal worker and he can make you extensions, or, just buy the new GTX Qtr Panels and have a body shop replace them. I do this stuff all the time, widening qtrs on a 69 Camaro so it looks stock but put it next to an original and BAM the differences are 'there'. I have not done this modification to any of my 928s and I will not undertake such a modification because I like how homage was paid to the 70 Chevelle by Porsche engineers on that area of design. And, the stock wheel well will hold a nice fat tire wheel combination in stock form. Fender rolling is damaging, and, nothing is easy and cheap. Do it right.
The rolling of the inner lip (my goal) turned out perfect. There was no unwanted metal deformation and the paint that cracked was on the bottom edge of the fender lip. I filled the loose paint edges with Jb weld and then applied touch up paint for future durability.
^^^^ I run those wheels and by far my favorite. Super easy to clean also. To be sure of clearance i put one side of my car up on the side walk curb in front of our house in order to nearly fully compress the suspension on that side. No issues with rubbing. When compressed the top of the wheel actually moves inboard in reference to the top of the wheel arch. When sitting parked i can fit a pencil between the sidewall and the lip at the tightest point.
im looking to get another set and powder coat them bronze metallic.
(photoshopped color on the wheels).but you can see the Car perhaps a sits a bit lower also. No issues.