I was reading in one of my cycling books about the benefits of drafting. I thought I would post some of this info since some folks are somewhat skeptical about "old guys" riding at 30mph.
The book I have says you can save up to 26.7% drafting in a paceline. I just saw another bit of research when I did a search online that says you can save up to 33.7%.
Using the 26.7% number: if the group is cruising at 30 mph and you are tucked in nicely drafting, then you would be putting out enough power to go about 22 mph if you were alone.
I try to alway keep this number in my head especially when the group exceeds 30. Another important thing to remember is to not be looking at the speed on your cateye. Just shift and ride as usual. You should only be watching cadence if anything.