I wouldn't put wider tires than the stock sizes otherwise it will just look like the sidewalls are bulging out. If you want to go wider, you need wider wheels.
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2005 Black GT Whipple HO | Oil Seperator | Steeda Tensioner | 93 dyno tune | Autometer Cobalt | GTAs
2006 GT Steeda Sport springs, FlowMaster American Thunders, Pypes Off-Road H-pipe, Tunable Induction CAI, CMCV Delete and MotoBlue UDrive pulleys with BamaChip's 87oct tune.
294.29RWHP and 316.65RWTQ= Before the Deletes and Pulleys.
I have the Pirellis on 18x9.5 cobra replica wheels out back, @ 285/40/18. I have the ecstas out front same wheel, @ 255/45/18. They rock and work well together the rims are abit heavy.:thumbsup
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'06 GT Vert Torch Red/Black top/int, IUP, Polished Bullits, , PioneerInfinityMTX Sound System, Tuneable Induction by DeMolet, SCT2 93 Octane Tune by Brenspeed, Red Calipers, Steeda Ultra Lites, Shifted by Hurst,
You can go to the Tire Rack web site and look at their chart to match tire width with rim width. You do want to stay within range or the tires lose lateral stability (cornering ability). For my front 245 tires I went with a 9" wide rim. For my rear 275 tires I went with a 10" rim to assure proper sidewall angle.
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'06 GT, manual, Satin Silver, lt. grey cloth int., IUP.
MODIFICATIONS - Goodyear F1 GS-D3s; 245/45 on 18x9, 275/40 on 18x10| Progress Technology springs & sway bars| Steeda adj. panhard bar, panhard brace, front control arm brace & bushings| strut tower brace| Tokico D-Spec dampers| CHE adj. LCAs & BMR relocation brackets |drilled & slotted front rotors| C&L intake| BamaChips SCT tune| FRPP 4.10 ring & pinion| Pypes off road H-pipes| FRPP GTA mufflers.
I come fro the Jeep world where we cram the biggest tire on the smallest rim...I have a 35/12.50 on an 8" rim...I still have to learn all this "new world" stuff lol
I have 255/50 tires on my stock rims. They fit great and the slightly larger tire size is noticeable. Some folks on the boards have put 275/50 tires on their stock rims, although I think that is pushing it in regards to safety. Do a search and you should find plenty of posts and pictures. If you can't, let me know and I'll post the links. I should have some pics of my tires in my gallery.
I just mounted two Kumho Ecsta SPT 255/50/17's on the back today. I wouldn't go any wider than that on the stock rims. I thought about the 275's but looking at it, it would have bulged bad, at least with the Kumhos. I tried pushing it around a little bit, but no squirm. I am pleased. A lot more grip than stock.
Been running a used set of Nitto 555 in 245/45/17 on the front for nearly two weeks now, no problems.
Folks, please be aware; that bulge isn't just a cosmetic problem. It's also a safety problem. Sidewall angle affects the ability of the tire to grip the road when cornering.
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'06 GT, manual, Satin Silver, lt. grey cloth int., IUP.
MODIFICATIONS - Goodyear F1 GS-D3s; 245/45 on 18x9, 275/40 on 18x10| Progress Technology springs & sway bars| Steeda adj. panhard bar, panhard brace, front control arm brace & bushings| strut tower brace| Tokico D-Spec dampers| CHE adj. LCAs & BMR relocation brackets |drilled & slotted front rotors| C&L intake| BamaChips SCT tune| FRPP 4.10 ring & pinion| Pypes off road H-pipes| FRPP GTA mufflers.
Here's pics of a stock Pirellis vs 255/50/17's Bridgestone Potenza. I am very please with the Bridgestones.
What is the specific name of the Potenza tires shown on your car? They look great!!!! Potenza tires have many different types depending on your driving needs.
Time for new tires.... 
