I would love to have a dead flat, 20 mile stretch of road to ride to see what I could cook up for an average speed but there's nothing even close to that around here. My fastest solo ride (about 30km) is just a tick over 20mph but includes multiple stop signs, stop lights, intersections that I have to slow for, etc. I do know that I wouldn't really want to push any harder than I did on that ride. I almost never get to ride in a group or even with a partner so I have no clue what sort of speed I might be able to push in a group setting. One of these days I'll tag onto the local shop ride and see how fast I get dropped.