This is from one of our local bike club web sites.
A.....For very strong cyclists: 19-20 mph avg.speed, few stops, hills likely. Slower riders will be left behind
B.....For strong riders: 15-17 mph avg. speed, few stops, hills likely. Will try to wait for stragglers.
B-...13-14 mph avg. speed, stops as needed, some hills to be expected. Ride leader will wait for stragglers.
C.....11-12 mph avg. speed, stops as needed, some hills to be expected. Group will wait for all stragglers and assist those having (non-major) mechanical problems.