will 255/40's fit the factory 17" wheel? I know someone here did 255/45's I think. would 40's be harder to fit due to the smaller sidewall not being flexible enough? got my snow tires on (YES) and now am now looking for replacements for the spring.

That's an awful wide tire for the stock wheel size; your tires would be 2.5 inches wider than the wheels. (255mm is 10 inches; the stock wheels are 7.5 inches wide.) If I've remembered my trig correctly, you're talking a sidewall angle change of ~2 degrees on each side (15 degrees off vertical for 45s and 17 degrees off vertical for 40s), so if 45s will work, 40s should work, too.

so will they fit??

I think ls4me had 255/40s on his, but he went back to 245s because of rubbing problems. Beyond that, I don't know. I personally don't like having 25% of my tread unsupported by the wheel cross-section, but I'm a bit conservative that way.

I have 255/45/17s on my 17s, no rubbing at all, looks great.

hylife do you have the factory 17" wheels?

sean, i have 255/45/17 on the stock 17" rims. no issues whatsoever. they were difficult to mount on the rims because of the width, but we got them on. imo, 40 series will be even harder.

Yes I do, I have the non chrome 17s on almost new tires, but I am keeping the untill I sell the car.

hylife do you have the factory 17" wheels?

My friend barely slipped 275's on his 1998 Mustang GT (8 inch wheels). I'd go for 255/45 R 17's because of protection of the rim, and to my knowledge they are cheaper. My friend had to silicone them to the rim.

Even if they fit, you need to be concerned with how the car will handle. By way of example, I put some 275-40-17 tires on my Mustang Cobra's stock 8" wheels and the car handled terrible!! I upgraded the wheels to some '95 Cobra R 9" wheels and the Mustang handles great.