New wheel and tire ?

[Slammed Vette]

Been looking at these:http://www.kmcwheels.com/kmc_images/...s/dime_big.jpg
Was wondering if someone can give me some help on sizing.I'm going with 20"but,backspacing and offset are like a foreign language to me.Don't want them to stick out too much(if not at all).Going with either Nitto Mud Grapplers or Toyo MT's in 33/12.50's