Dan, a 180 will fit your rear rim safely. I've been riding my 6R with 120/70 front 180/55 rear Dunlop 207Gps. Since the Stars came out, my vendor sells GPs for 285 m&b, which is cheap. From now on I am only going to ride with 170/60 on rear, I have learned that the 170 has the biggest contact patch among 180,170,160 tires. One thing you need to worry about is the shock resevoir by your passenger peg, if you botton out, the tire will hit the banjo bolt, so either relocate it, or zip tie it closer to your passenger peg. I have always had 170 on rear ever since those OEM bridgestones.