http://www.kreuzotter.de/english/espeed.htm

you can punch in a wattage, which then gives you a speed based on all the other vairables.

I just put in 450 watts, selected "hands on the drops", and left all the other variables, which gave me a speed of 28.9mph



or are you using this one?
http://www.analyticcycling.com/ForcesPower_Page.html