285/35-18 rear tires

The Pilot Super Sports are 11.1" wide in 285/35-18 (in photo) (228) Deep Purple - Highway Star (Official Vinyl Video) - YouTube
The Pilot Sports 4s are 10.1" wide in 285/35-18
Michelin custom makes Super Sports per manufacturers request, hence the different specs in the same size tire
Above right fronts, 255/40-18 Pilot Super Sports, 8.7" wide on 18X8.5" wheels
All 4 are ~26.0" diameter

BMR G8 bolt in Butterfly braces, then welded keeps those 285/35-18's planted
BMR Suspension SFC013 - Subframe Connectors, Bolt-on - 2008-2009 Pontiac G8
For the front:
2008 - 2009 Pontiac G8 Chassis | Strut Tower Braces | BMR Suspension
Very confusing nomenclature: This is NOT a conventional Strut Tower Brace but BMR didn't know WHAT to call it. Holden designed it for the G8 but never produced it. It reinforces the strut towers about 6" below where any STB would mount. Here it is installed with a Whiteline real STB. Optionally, it can welded to the sub frame, as this is. Holden originally called it a Subframe connector, as did BMR, but that was ultra-confusing considering the SFC013 shown above, so BMR renamed it, adding more confusion. No matter, it was designed to eliminate the imprecise steering caused by the strut towers flexing, even when the car is new. It does that even with 255/40-18's. It mounts in addition to the welded subframe at the power block on the driver's side and the ABS module on the passenger's side. No traditional STB can eliminate the strut tower flexing, but this can.