Limitations to training on singlespeed?

If you're setting out, in a group, to do,say 60-90 miles at a steady 17-20 mph then a single gear in the 65 - 70" range should be perfectly adequate from a speed point of view

Yeah, but you know some bugger is going to come out on 76" and call "sign" just as he comes flying through from the back at 25mph