I run a 15x 8 x 4.75 B/S rim. Its a special order torque thrust, CL205 series. The tires are 275x60s. No rubbing. IF I remember correctly, I got about 1/2 between the tire and the leaf spring, so there is room to go.
http://www.hbci.com/~chriss/PhaseSix/RRear.JPG
http://www.hbci.com/~chriss/PhaseSix/Rear.JPG