I don't know nothin about tire sizes. I need to ride up and down a mile long dirt/gravel/rutted road and need better traction than street tires on my K100rt. I have Michelin Anakees on my Triumph Tiger and love them for dirt roads. Wa nt to put some on the BMW but can only find 130/80 17 when stock rear tire is 130/90 17. Will there be any problem with the smaller ratio? Same issue for front tire. Thanks for any advice.