+1 on "watts is watts". Everything else is just...not.
My post was a little more of a commentary on the area I live in - where a "flat" 28 miles still has 1,800 feet of climbing. Did like the 18 though :-). If you live in flats, for comparison, my average for the year is 16.25 mph. When i rode down in Marina del Rey (dead flat circuit) earlier this year I averaged 20-22 depending on how I felt, wind, rabid skateboarders, etc. That went up to 27-28 when riding with a fast early morning pack I kept running into (not literally) :-).