In my club the average ride speeds vary by time in the season as well. I usually ride the B or B+ rides just cause they seem to be the most pleasant for me. In April a B ride might average 13-14 mph and by Sept. it could be up to 16 mph. As stated here by others, the rides are published on Friday with distance and anticipated average speed. The rides are on Saturday morning.

FWIW, yesterday I did my 17.4 mi. TT (1200 ft climbing) at 16.4 mph. By Sept. I expect to be close to 18 mph. At least that's the hope. For my club I'm pretty much a B/B+ rider.