The 90/90 is bigger because the stock Dunlop is an MH90 which is the same as an 80/90. I run a 90/90 Avon on the front. I also have a Screamin Eagle Dunlop 502 180 on the back. Just a pully spacer was required and I removed the lower belt guard. You can notch it too if you want to keep the guard. The 180 is a nice improvement in looks. The 170 Metzeler will actually be a little bit narrower than the stock 160 Dunlop, but it is a great tire.