It's important to emphasize that riding with a group can greatly increase average speed. The aerodynamic effect is huge and increases exponentially with speed. I find that simply drafting one rider can increase power by the equivalent of 3-4 mph. Thus, for the drafter, the power output for 18mph is more like what it would take for 14 mph without drafting. Increase the size of the group and the aerodynamic effect is greater. So, when I here bragging about group rides averaging 18 or 20 mph, I'm not too impressed.