44x17 for me, so 69.5" at 30mm tires. That puts my sweet spot at 18mph, and up to 20.5 is comfortable.
I rode a 200k Tuesday with ~4k feet of elevation, with a tailwind all day. I can manage 150rpm/30mph for a brief period on a descent, and I was at that limit several times. On the other end, I had to dismount and walk one short steep hill, and I slogged up a few others. On some of the tailwind flats, gearing was limiting my speed. Overall, I think my gearing was a pretty good compromise.