I'm currently running a flip flop hub with the fixed side at 17t and the freewheel at 19t...49t chainring. It's moderately hilly where I live and let me tell you, I was cooked after pulling that trailer for 3 hours in 90 degree heat. http://connect.garmin.com/activity/39934922
Looking at the picture I see that I've changed my setup quite a bit since... I've changed the crankset to a MTB setup with lower gearing (in that pic I have a 105 crankset w/ a 26 tooth small ring installed), I've changed the bar tape to sewn on leather, raised the bars, I've added a FOURTH water bottle holder (in addition to a bladder in one of the front bags, I hate running out of water on 100+ degree days!), and of course I lose the tent (the green stuffsack) and pad (blue roll), but it's replaced by underquilt and hammock, so not really that much smaller.