Likewise, I have 20x10s in the rear and I have 285/30/20 in the rear. If you go on tirerack or american muscle select rim size and they will recommend tire sizes but be prepared for the price. I just put rubber all around and it cost me $1100
20x9 and 20x10:knock off brand (Toxic) 5 spokes
255/30R20 and 295/30R20: Proxy 4's by Toyo
Sorry I wish I could tell you the offest! But there is pleanty of room on these cars for wide tires!