I am gonna run 33x12.50 SSR's on 15 x8 black rockcrawlers. What backspacing should I run to keep the tires as much under the fender as possible without rubbing?

 

Does your rig have a suspension or body lift? It depends on the actual tire size. My rig has the OME suspension lift, 285's tires on 16x8 with 4 7/16" backspacing. It fits right inside the fender perfect, but I had to hammer on the inseam flatly on the firewall inside the front fender to clear out the tire rubbing. It usually does not get any rubbing under a normal driving condition except during a hard core at the off-road trail, you might see a slightly rubbing under a hard suspension punch. But, it was no big deal.