UPDATE: Commute home made me feel much better about myself, as the cyclometer was now producing accurate readings after my calibration tweak today.
Amazing how self-esteem can be tied into something like this.

Tonight on the commute home, in the dark, in the rain, my average over 16 miles was 15mph, and my maximum was 25mph... and that's on a singlespeed bicycle. The moments when I hit 25 in the rain in the dark felt dangerous to me so I backed off.