I forgot the another VERY important point, and that is drafting.

If I'm in a big group, I can easily ride at 20 too. I actually did a little experimenting on a very long, straight stretch of road (from Alamosa, CO to Salida, CO - just a very, very slight uphill grade, but not enough to be even noticeable) Riding by myself, I could push it to 16 mph. Tucked behind my rather thin boyfriend, we could up it to 18. Add just one more rider, and we could hold 20 for quite a while. Got behind a great long paceline, and we were holding 23-24mph for an hour. But, after 40 miles of this, when I dropped back, I could only manage 14-15. So many factors are involved, that trying to set a generic '13mph is slow' statement is just impossible.

The only way to know would be to put us all on the exact same TT course, no drafting, and at the exact same time, so the wind wouldn't be a factor, all on the same bike, so weight wouldn't matter.