Your numbers help, especially 0.29 mph. I weigh about 170 lbs. I'm not terribly strong or terribly weak. I don't often ride at 18 mph, though. I think my typical cruising speed is about 14 mph.

I put my numbers in. I said my bike weighs 30 pounds, since it often does, or more. I don't know anything about my frontal area and air drag, so I left the default values in. So I need about 100 watts to keep that speed up. So hmm, still, the dynamo, if it truly sucks 6 watts, is slowing me down by 1/4 mph. That is truly imperceptible.

If I take a century ride with a bunch of friends, how much more tired will I be than they are at the end of the day? I'd imagine lots of other factors will bear on that more than the drag of the dyno hub.

Any thoughts about the accuracy of my 50% efficiency guess?

I don't even know if the electrical output is actually 3 watts. What kind of rating is that? Average? Based on some assumed speed?