For good or for bad I'm obsessed with numbers related to my riding. Before the ride I plot out my expected time on a control by control basis. It gives me something to shoot for and I'm usually within 20 minutes on a 300K or 400K. I know my average speeds, I know how different terrain, time of day, distance, etc affect my speed. I only have personal data out to 400K though.
My question is what kinds of reduction in speed have others seen or measured in multiple day events. Measurements of my own long rides suggests (very roughly) 0.3 mph fall-off in speed for each 100K. So for a 1200K started at around 14 mph I'd finish at around 10.5 mph. That sounds reasonable. I know there are many many variables that make a linear extrapolation of speed unlikely to hold up over a four day event but I'm just looking for general trends.
So what are your experiences with your own speeds over multiple day events?