Pontiac G8 2008-2010 - G8 on 18's Question

I am helping a buddy out here with his car. The car is a completly stock 2009 G8 GT. He wants to put 18x9.5 wheels (already knows what wheels he wants) with 275 wide tires on all 4 corners. The question is:

The wheels he wants only come in a 18x9.5 = 40mm and 56mm offset

Will the wheels fit on the car (stock height, loaded dow,. and lowered 1-1.5") and if they do which offset for a 275 wide tire, AND should it be a 275/35/18 or a 275/40/18????

Any help is appreciated. I am also looking on a couple G8 forums for answers too but prefer to not have to become a member on them to get an answer or post a thread.

Thanks

275-40-18 and the 56mm will be great, should be almost like stock, which are 48 or 46 i cant remember without a goggle search lol

So 56mm in the rear? From what im reading a 40mm offset in front is ideal if you plan to upgrade the front brakes to a CTSV setup which he probably will.

That's a high offset in the rear. As said, it should be fine, but I'm not sure how much clearance there is on the inside.

9.5's on the front with a 40 offset should be okay, but it's cutting it close..

There is a guy on the G8 board running Conti DW's 275 on the front with 41 offset, but Conti's run narrow, apparently.