Someone, correct me if I'm wrong but using displacement maps would probably require just as many polys as (if not more than) modeling the tire since displacement maps usualy need a high number of polys to shape the geometry. Although with subdivs you can generate those polys on render so you don't need to worry about them during modeling or animation.

I'm not positive about your question, but I think 127,127,127 is neutral because 0,0,0 pushes inward.