Originally Posted by
cafzali
If you want to have as good an idea as is possible without actually going out and chancing it, try a solo ride for the length and speed that you expect for the group ride. You'll expend about 20 percent more energy riding solo than you will in a group, so if you can do that at the advertised speed, you know you're good to go. Of course, this requires having a cycling computer where you can measure your time, distance and speed.
This would be true where it's a steady speed. But many rides have a very mellow, neutral roll out and then you hit the start (which by the way will be a signpost or a guardrail or a tree or even a line where the pavement color changes) and then it's on like Donkey Kong. If you're not ready and able to match the surge and get in the draft, you'll be off the back before you know what's happened.
Which is why I recommend asking what's up at the start and being self-sufficient in any case.