If you're willing to have wheel and tire stick out from behind the vehicle than anything can fit.
The wheel wells on the G8 are rather small, and over 285mm with proper offset to sit in the fender and not contact anything starts getting hard. Rolling fenders usually gives a few more MM to play with, but you'll need to seriously pull fenders to fit something over that.
2018 Red Quartz Tintcoat GMC Sierra 1500 Denali Ultimate
2016 Jungle Green Metallic Chevy SS - Sunroof/Spare/Stick *Traded 2/3/18*
2011 Radiant Silver Metallic CTS-V Wagon - Corsa Axleback, AirAid Intake *Sold 7/7/16*
2008 Panther Black Metallic G8 GT *Sold 6/11/14*