Just the other day I saw a roadie just ahead of me that I just happened to be following for a little while, so of course I tried to catch up. I noticed that I was closing the gap more on the slightly hilly sections because my fixed gear forces me to work harder to maintain a decent speed with the higher gearing, and of course it probably took lots more energy. On the flats I couldn't spin fast enough, so he gradually widened the gap. It's a shame cause I just wanted to say hi