285/35/18 is the widest you can go and still be within the recommended wheel width with stock 18 x 9.5 rear tires. But on one of these cars, you probably do not want to run the minimum wheel width for a tire. I would choose 275/35/18 or 275/40/18 instead. You can see the recommended and measured wheel widths in the specs section for a tire at the Tire Rack website.
I chose to run wider wheels, 18 x 9.5/10.5, which I will fit with 265/35/18 and 285/35/18 tires. One inch wider wheels and nearly one inch wider tires.