so i'm needing new tires. heard great things about the shinkos. problem is that they dont make one in the same size that i have right now (150/90-15), but they do make a 150/80-15. that is a shorter tire than i have right now, and i actually want to go taller. at 70mph i'm right around 5K rpm, and i'd like to drop that down a little bit. anyone have any suggestions?