Is your FX a triple or double? When I had my FX, it was a triple and for some reason, I rarely went all the way up to the big ring in the front, keeping me from going faster. It was kind of laziness on my part, because it seemed like as soon as I'd hit the big ring, i'd need to shift down again pretty quickly. I used to ride my FX on group rides where everyone else was on roadies. I would fall behind, but I could keep up.

Enjoy your bikes !