Would a 275-35-18 fir on a stock 92 RS?
I work at a discount tire and happened to get some really decent Potenza RE 71R tires from a kid with a mustang.. I was looking online and saw someone running a XXR wheel on his third gen with what looks like a 235 or 245 ish size. Thanks in advance.

 

You're not gonna fit a 18 foot tire on a thirdgen..

 

He's right, 18' won't fit! ....but if that's just a typo,LOL, and you meant 18", then yeah.....they'll fit, assuming everything else is the same.

Guys are regularly putting 275 width tires on 0mm offset wheels, and having no issues. Some have very minor rubbing up front, some do not. Either way a .25" spacer solves any issues. ...as for height, a 285/35 is is 25.6" tall, which is right at stock height. Again, no issues.

The problem arises when the wheels or the rear are not stock offset/widths, etc. etc. ...if you haven't changed that, then that tire will fit.

 

Yes, it will fit front and back if you have correct wheel width and backspacing. 18" usually clear the steering tie rod so should be able to run 9" or 9.5" width front and back.

My guess for wheel specs are,

18x9 and 5" backspace (0 mm offset)
18x9.5 and 5.5" backspace (about 6.5 mm offset)

These numbers will not allow the tires to tuck though, you'll need to have stiff springs to keep the tires off the fenders. You'll want to measure up on your car to make sure it works, especially the 9.5" wide. Cars with ground effects have more issues with front tires bumping into the back side of the inner fender liner.

 

I ran 275-40-18 front and back with only rubbing a full lock, like parking lot turns. I auto crossed with 275-35-18 with no rubbing. That was with C5 corvette wheels . The rear was 4th gen width with no spacers and the front had a big brake conversion with 1.5 inch spacers.