Air Pressure and Nitto Drag Radials

[Joe50]

I run 275/40/17 nitto drag radial on 9" rims on the street everyday at about 15 psi. I was noticing that im wearing out the center 3" of tread, but the outsides of the tires still have tread. Generally this is a symptom of overinflation, Right? Am I running the tires at too low of pressure and having the opposite effect? I dont think I can go much lower without risking the tire coming off the rim in a sharp turn. Anyone else having this problem? Any suggestions? What air pressure is everyone else running on the street?

Thanks
Joe50