Distance; typical, fastest, slowest
100; 4.5, 3.75, 6
200; 11, 8.5, 14
300; 17, 14, 20
400; 24, 20.5, 27
600 36, 34, 40
The faster rides tend to be calendar events on flat terrain, bang on distance (e.g. 100k is 100k route not 116k), in (late) summer. In the list above, ones like the San Ann Fairy "flat" rides or the Mildenhall Rally rides.
Slower tend to be the opposite characteristics.
Anything with night riding and my average speed drops dramatically due to genuinely lower speed (despite it "feeling" the same - due to loss of visual cues about speed), sleep and navigation (this latter points is helped by GPS with backlight, I found the winter just gone). One of my "goals" is to ride a 400 in less than 20 hours at some point in my audax "career" and it is proving challenging.