There are so many variables here - wind conditions, road surface, personal fit level for the day, how long are you sustaining that speed...
but since you asked I have hit 33 with a tailwind, on a really nice section of pavement and sustained it for about 3/4 of a mile.
I do this route 3 - 4 times a week - usually hit in the high twenties - unless I'm feeling crummy and then it's 23 - 24.
Now downhill on the tandem - 59!![]()
Veronica