You know, if I think I'm going to be slower than the group, or I figure that out during the ride, what I'll generally say is "If you folks get way out in front of me, feel free to drop me." That gives me enough wiggle room to do as needed without making any big deal out of it.
The dynamics of a group, competitiveness--drafting--conversation--will often help to pull you to higher speeds that you wouldn't normally achieve so it's hard to make a judgement at the beginning. When I ride alone, I can average 15 or 16 mph, yet New Year's Day riding with a bike club, we blasted along for 3 hours averaging 18+.
So, sometimes I surprise myself and keep up and others I'll finish alone.
Sheldon
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