I did my FTP test on a flat unobstructed 20mile long course (out and back) on smooth pavement at sea level with very light winds today. I'm making progress and all is well.

As somebody who'd like to eventually do the local time trials, my question is more about a curiosity I have wondering why my speed numbers are so much lower than what formulas might predict.

Obviously there are a million variables and I understand equations and their inputs are all approximations, but the data spit out from http://www.analyticcycling.com/ForcesSpeed_Page.html is so far off from my numbers I wonder if I'm missing something. In fact, it estimates I should be able to go the speed I did at 150 watts, which is a huge delta between what I actually put down.

Even if I enter worst case scenario type numbers I can't get the speed that low in the calculator.

The FTP test results:

http://connect.garmin.com/activity/123071041

Avg power was 239w WITH zeros. I'm 6'1"@159lbs. I'm not wearing MC Hammer pants or any baggy clothing. Was in either the drops (5%) or clip-on aerobars (95%). If I ride solely in the drops I'm about 0.5mph slower.

So analytic cycling predicts 24.7mph at 239w, compared to my 20.7mph at 239w. 4mph is a huge difference at these speeds. Anybody have input?