Well posted speeds are an indication of where you should fit so they should be reasonable accurate. We had a situation in an group I was riding with, it was not no drop and the speed was listed at 21-23. When the speed would go over that some people would start yelling even though the majority of the group had no problem with the increase. So they broke the group into two groups and the second group would be a sweap group to pickup the guys falling off the first group. Even that did not work as there were always 10 or more riders who continued to push the pace up in the lead group to the point people started complaining.

They finally just defined this group as anything goes, no posted pace, so the pace would vary week to week depeding who showed up and no one could complain. Actually made the ride much more fun.