What size tire are you trying to go with? On the 19's, a 245/45 is perfect so on a 20", a 245/40 is perfect, based on a 8" or 8.5" wide rim. The diameter of a 45 series on a 20" rim ( unless on 225mm wide) may be to large definitely when lowered, more rubbing problem will occur, I already am with in 1/18" when turning, I have a lot of rub marks, I just don't want to see you regret your decision.
Later
Steve