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Can they fit the GS properly or not is the first question? Second is what is the widest tires i can put on these? 245/40/18 Front and 255/40/18 Rears is the rim width to skinny?
235/40 (optimal) or 245/40 should be fine. I would NOT do 255. It may mount but I would not feel safe. I would stay with one size, though. If you were going to stagger the tires, it would have been better to stagger the rims appropriately to maintain a consistent wheel diameter.