i just change my gears so i'm at ~90 rpm at my cruising speed. was running 48x18 for a while, just changed to 48x17.

i would just figure out what your common speed range is (leave out the sprints and heavy traffic situation) and go from there.

also depends on whether you are riding hills, etc.

i'd say try a 16, 17, or 18 on the rear and keep the 47 up front.