I get the best mpg on the '05 K12LT ~ 45, the 91' K100RS 4V yields ~ 40 (and I ride it most aggressively) and the '85 K100 Standard (no shield) is ~ 38. I feel like a sail on the '85 with it's upright seating position. My weekly route is the same for all bikes, I'm primarily a commuter, and I run Shell or Chevron Premium. I used to run Regular in the '02 K12LT as it started better on Regular when it was winter - not that we get much of one here in Alabama the Beautiful, but I worked Second shift and would get on to go home between 10 pm and 2 am. My rule was, if it were above 40 degrees when I left home at 1 pm, I would ride. It might be below freezing when I left work for home (heated vest are NICE! and the heated seat doesn't hurt either =-}). The '05 K12LT doesn't seem to have the same issue but I no longer work Second shift to know.