Depending on the type of riding you do, average speed may or may not be very variable--like people have pointed out, there's traffic lights, weather, road quality, etc.

WHere I saw the difference between my FX 3 and my road bikes was in the top speeds and the amount of time I could sustain them. Over a long ride, I might have a 1-2 mph bump in average speed, but my top speed (level) could be 2-3 mph (in the mid-20 mph range). As has been pointed out, this makes sense because it's at the high speeds that the aero effects really kick in.