From Princeton Freewheelers:

MPH's are for 1) avg. speed, followed by 2) flat cruising speed:

Ax
23/ Up
26+

A+
21 - 22
24 - 25

A
19 - 20
22 - 23

B+
17 - 18
19 - 20

B
15 - 16
17 - 18

C+
13 - 14
15 - 16

C
11 - 12
13 - 14

D+
9 - 10
11 - 12

D
8 - 9
up to 10

When I first started riding with a club, I found out that "B" rides were extremely dangerous. You would get a lot of very fit people that really didn't know how to ride a bike in a group. I pushed ahead and moved to B+ and A rides, where the rides were much safer due to the experience of the typical participant. When I was starting to lead Ax rides, I could keep up with avg. Cat III racers, and Cat II's would ride away from me at any point. The speeds of the higher category rides assumed effective pace lines.