i have a 2005 gt with stock suspension and i have the stock 18's on it... im lookin to get wider tire in the rear, what kinda tire sizes can be put on this thing?

 

Go across the top of this list.Maybe you could find the right size tire recommended for your 18's.I'm going up to the 255/45/18's for my 18x8.5 stockers,just a little wider is all.
http://www.bfgoodrichtires.com/specs...-a-kdw/22.html

 

I've got 275/40/18's on my rear when I need new rears though I'm thinking about going with 305's or 315's

 

would a 275/40/ r18 fit at all on a stock 18(18x8 i sapose) rim under the car or is that gonna cause rubbin?

 

It's not whether or not it would rub at all, and for the record, you would be fine.

It's not a good idea due to the width of the stock wheel. Being only 8 inches wide, a 275 would bulge a bit, and actually reduce your contact patch. I wouldn't run any wider than about a 255 on the stock wheels.

Once you upgrade wheels and get like a 9 or even a 10 inch wheel, THEN you can start thinking about 275's and wider.