What are the widest tires you can get on the rear of these cars with the 16x8 GTA rims?

255s

I don't know if this applies to 1988 GTAs, but I once put 275 50-series tires on a 1985 Z28 (with stock wheels) without any problems.

I use MT ET street 12.5 inches wide on my TA.
Its the equivalent of 317's....

How far did they stick out? Or did they at all?

255s don't stick out, they are still within the upper fender lip.
275s also fit but the only guy I know who has them had to roll the lip under - and he was also using aftermarket rims with a backspacing to make it easier to fit without rubbing the inner wheel well.

I have 255s all around. In the front they only rub at full turn and only when the car is being turned in the driveway, otherwise they don't make contact. The 255s in the rear also fit fine, but I have noticed that under extreme turning when body roll sets in they will rub the inner wheel well, but only slightly.

Hope that helps.

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1991 RS LT1
every imaginable bolt-on, and an all-forged 355 on the way with an ATI following in '01

2000 C5 Coupe
black/black 6-spd, optional polished rims, 'mostly' stock....

"So what DOES the back of a Mustang look like?"

255's all the way around or you can go 275's out back.

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TPI 305,
Gutted airboxes,
gutted MAF,
K&N filters,
Corvette Servo.
14.9@93mph

"Speed kills, wanna live forever, drive a Ford."

Is'nt there an undesireable amount of sidewall present with the wider 255's,that adds flex & interferes with handling, plus also looks a bit out of place?