If you define average as time while moving of distance traveled (i.e. rest and stop time is not counted) then in my club:
A: 23
B+: 19-20
B: 17-19
C:14-15

Advertised ride averages are typically 2 mph under what the pack travels at.
A rides are - every man/woman for themselves
B rides are - we will wait for you at rest stops but thats it
C rides are - we will wait for you at intersections and major turns but don't expect company while you are riding.