Sorry if I'm stating something that's already been posted -
Are you guys waare that the G8 was designed to run 9.5 inch wide rear rims?
They are standard on the HSV range, still 8 inch rims on the fromt but the rear run either 19 inch or 20 inch rims with 275 x 35 or 275 x 30 respectively.
You can find out offset info by contacting an OZ tyre and rim dealer like www.bobjane.com.au or www.tempetyres.com.au