I just mounted the Avon 100/80-16 up front and the Michelin CityGrip 110/80/16 on the rear. The Michelin is a front tire, not available as a rear. Since it's NOT a radial, it's perfectly acceptable to mount in reverse rotation and use it as a rear. In fact, before motorcycle tires came in Fronts and Rears, that's what was always done. The 110/80 rear has a larger circumference than the stock 100/18, and necessitated slotting of the mounting holes for the hugger (internal splash guard under the rear body work. 2 minutes with a Dremel tool.) Clears nicely, feels much better handling. The 100/80 up front reduces the speedo error substantially. According to GPS, it's now about 4 MPH optimistic across the board, rather than the 14-15% variation from actual with the stock front tire.
(If you don't want to enlarge the rear tire diameter, you could try the 110/70-16 which is more readily available. If the tire spec's out true to numbers, it's diameter would be slightly smaller than the stock tire, though wider. No personal experience with this one, though.) 120's are definitely too wide.
