Everyone's right. It's a bit of trial and error. I have a flip-flop hub with a 44T chainring and have a 17T freewheel and a 16T fixed cog. I live in DC so it's pretty flat (besides going up Capitol Hill) and I love the 44T/16T setup for the city. The 44T/17T was fine, also. However, if there's a lot of hills where you are, go with more teeth on the rear.