the factory wheels come in 18" i am switching to 20" do i have to worry about anything else

 

Definitely yes.

Diameter is only 1 measurement, and IMHO 20's are too large for this car and for NY pothole filled roads. You also have to factor in width, offset, bolt pattern, lug nut taper, weight, forged vs. cast, 1 piece vs 2., materials and tested strength. It's very easy to get a crappy wheel that doesn't fit or perform well but it's hard to find just the right one. Been there man...

Why so big? First post, did you just get the car?

 

i run 20's and they aren't too big at all.

depending on if you're going staggered you will probably want a
20x9 et35-38
20x10.5 et45

if you are awd and don't want to stagger you'll want to do a
20x9 et35-38

 

I'm in NJ and on 20's. Et 38, tire 245 35 20 all around. I have a 14 AWD. Just watch out for the potholes best you can and get tired inspected at service intervals .

 

dubs are fine on the GS, actually looks great!
there are lots to consider sure anytime one upsizes the factory rolling stock but fitment should be ok (assuming the offsets and tires sizing are optimal).

I have an old adage that most cars can accept wheels 2" larger than what comes with it stock.
so if a car come with 18s from the factory, then 20s will fit just fine, and I've seen it done well.

 

Sorry for the late response, thanks a lot timmy0tool, Twidge, mikesd1980 for the comments the is actually riding nice and I been swerving potholes no matter what state and no matter if its rims or factory wheels im a NY driver that loves NJ DRIVE the movie so act like you know. Here is a pic of my baby

 

All haters could do is watch me roll BYE.........

 

Look at those mother****ing wheels

 

I llke those wheels.