City riding rarely gets me above an average of 17, most of my commutes are 12-16mph on average (my usual cruising speed is 19-23mph).

City riding is hard to keep high averages, I don't want to run lights that bad and I usually brake WELL in advance of lights and slow-roll to them so I can get a rolling start, where possible - this drags the computer average down a lot too.

When I've done out of town longer rides, my averages are more like 17/18. My gearing was 46:17 until recently, I now run 46:15.

My averages change more if I catch/run lights than if I consciously try and go faster - as I usually always ride at the same cadence/speed unless there's some dickish wind in the play.