I have a 2006 GMC Sierra 1500 Z71. I am wanting to put some rims and tires on it. I am currently running a 3in body lift with the stock 17in rims and 285/75 toyo open country a/ts. I am wanting to go to an 18in rim with 33/12.50 open country a/ts or m/ts. The rims I want in an 18 come with a -12mm or +18mm offset. I want the tires to stick out farther than they do. My question is should I get the -12 or +18 to acheive what I am wanting?