Axis Angles & 305's in the Rear

[StratonAce]

I've got a base 2011 370z (no extras, no extra packages). It is not lowered, in fact, it is not modified in any way at all.

Currently there are 18 inch wheels on my stock Z, although I have no idea what their widths are. The stock tires are 225/50 and 245/45, front and back.

I was wanting to get some 20 inch Axis Angles, 9 inch in the front, 10 in the rear. And for the rubber, I was wanting to run 275 in front and 305 out back.

So, I went in to a local wheel shop and the guy there was working with me. He called his supplier and they could get the wheels for me. Offsets would be: front: +20mm / rear: +25mm

But the supplier told him that running tires that wide would not be possible without me modifying my fenders. He suggested I not go any wider that 285's in the rear unless I'm willing to modify the fenders.

Now I'm seeing people all over the forum running wider than 285's in the back. How are they running so wide? I'm wondering if they've modified their own fenders?

I was also wanting to lower the car in the near future, but if I'm already running into issues just trying to get bigger wheels and tires under the ride, surely it will only compound the problem by lowering.

So, can someone confirm for me the widest I can SAFELY go on a bone-stock 2011 370z? I was really wanting 305's in the back, but if I cannot safely go that wide, well then, I guess I just can't go that wide. But then, how wide CAN I go? I'd rather not have to roll the fenders or do anything else just to get the wheels and tires on, at least at this point. I just want some wide rims and meaty tires without having to screw up my car.

Can some one help me with some information? Thanks in advance.