The 2011+ Subaru WRX has great big bulged out fenders, just like the STI has had since 2008. We've been getting a lot of questions about wheel and tire fitment on these cars, and I've seen a lot here on NASIOC, so I thought I'd address it.
The bigger bodywork allows for the fitment of wider wheels and tires. Wider tires look cool, and more importantly, they can help generate a lot more lateral grip. HOWEVER, a lot of the questions I get are asking about fitments that are either too low of an offset, too wide of a tire, or some combination of the two. Oh, and there's the bolt pattern problem, too.
First off, some basics: The WRX has a bolt pattern of 5x100 and a hub bore of 56.1mm. The stock wheel is a 17x8" +53 offset with a 235/45R17 tire. That tire size means the stock tire has a diameter of 25.3".
That 5x100 bolt pattern means factory STI wheels WILL NOT fit, since the STI uses a 5x114 bolt pattern.
Now, if you want to fit larger/wider wheels and tires, you'll want to try to keep the wheel offset as close as possible to stock. I'd generally try for an offset near the stock +53. +48 is very common. Depending on how wide the wheels and tires are, though, the offset will have to go lower, or the tire may bump into the strut in the front.
From experience, a 9.5" wide wheel will work at around +40 (give or take a couple of mm). The lower offset allows decent tire clearance on the inside. Steering feel is slightly worse from the lower offset, but it's not terrible. A 265/35R18 tire happens to be within 0.1% of the stock size, and that fits pretty well. (You may have to do minor fender rolling at the back if the car is lowered. It depends on the tire...not all 265/35R18 are created equal.)
Here's an example of this fitment:
18x9.5 +38 Rota Option, 265/35R18 Michelin Pilot A/S
The car above is lowered on Epic Engineering springs. We rolled the rear fenders.
18x9.5 +40 Rota D-Force, 265/35R18 Kumho V700
That's our shop car above. Rear fenders rolled, lowered on Cusco coilovers.
If you still want more meat under the car, you can run a 275 width. We've done several cars with 275/35R18. That's a tight squeeze if the car is lowered. Rolling is definitely required in back.
18x9.5 +40 Rota D-Force, 275/35R18 Dunlop Direzza Z-1 Star Spec
The gray car has Swift lowering springs.
Our shop car with Rays G-Games 99B 19x9.5 +40, Cooper 2XS 275/30R19.
We have not yet tried 285/35R18, for a couple of reasons. One is that the tire is 2.1% larger than the stock 2011+ WRX tire, slightly over our 2% rule of thumb in terms of variance from stock. The other reason is that since the diameter grows along with the width, the clearance problem will get worse. I think that tire would rub at all four corners with any amount of lowering or suspension compression.
I realize I've used some STI cars as examples above, but the WRX bodywork is identical, so for our purposes these are still useful comparisons.
If you want absolutely NO fitment problems, go with a smaller tire, like a 255/35R18. Given an offset between about +40 to +48, there should be no rubbing, even when mildly lowered. Stock STI tires, which are 245/40R18, are also an easy fit.
I mentioned rolling the rear fenders. Customers often ask me about rolling the fronts. They usually do not need to be rolled, but even if you wanted to there's almost no lip up there to roll, and what lip is there incorporates clips that secure the fender liner to the fender. Get rid of those clips and your fender liner is going to have to go. We suggest retaining your fender liners. They keep road debris and moisture out of your car's inner bodywork.
Note that the bigger tires may rub bit of the rear bumper cover. We trimmed that back on some of the cars with 275/35R18 tires.
I see some people with the widebody cars running lower offsets, sometimes much lower, like +30 or +25. I DO NOT recommend doing this. Sure, it looks kind of cool (if that's your thing), but generally you are going to have to go with a SMALLER tire size to alleviate fender contact problems. What's the point of having big wide wheels if your contact patch is the same as stock? Secondly, as the offset gets lower your wheel centerline is getting farther and farther away from the axis of your steering. The steering feel gets messed up -- to me it feels vague and it tends to want to steer left and right on acceleration or braking, due to the wheel trying to pivot around the steering axis. This effect gets worse with lowering.
So, that's my experiences with stuffing big wheels and tires onto the 2011+ WRX. I hope this provides some help when you are shopping for wheels and tires.