reason i say 225 is ive seen it on my car and like
how it looks.
225 is not a great choice for winter on a Pontiac G6 (if I remember right). And running only 1 225 on the back would compromise the ABS I would think and tracking.
I've run some pretty sketchy vehicles, but even I would not run a vehicle like that. Get rid of that one tire!
Save your fat tires for the summer and don't buy 225/50/17. They are WRONG for that car.
Tortoise offers the best option because 225/45/17s are almost identical to stock, although they will bulge over the stock GM rim a little I would think.