I take a scout group on a week-long bike trek on the C&O Canal towpath most summers. That's a flat but unpaved trail, and most of the kids have hardtail mountai

I take a scout group on a week-long bike trek on the C&O Canal towpath most summers. That's a flat but unpaved trail, and most of the kids have hardtail mountain bikes because we also do trips for singletrack mountain biking. Over the course of the week, all the kids doing the trip for the first time get faster from day to day. When they start out and don't take my advice about tire pressure seriously, their sustainable cruising speed tends to be in the 8-9mph range. High tire pressure (and careful attention to ensure the brakes aren't rubbing) gets everyone up to at least 11-12mph depending on how hot the weather is. Drafting gets the group up to 14-15mph. So basically, in my experience on longer rides a low tire pressure will prevent you from getting up to the speed where aerodynamic drag is a big concern. And on hilly terrain you spend a lot more time either climbing at speeds far too slow for aerodynamics to matter, or on descents where you have no trouble at all overcoming drag to reach unsafe speeds.