This evening I did 10miles, and my polar s725 watch (I know, there's about +/-5% accuracy in them), showed an average of 303W. The time taken was 26.57min. Its an out and back course, so theoretically the average gradient is 0%. (In fact its extremely up and down, with total vertical climb of 180m, so I aim to push hard on the hills, and recover a little on the descents, as there seems little point in revving at >120rpm in 53x12). My weight is 76kg, with a bike wt. of 10kg.

I assume a CDA of 0.5 (I don't know if this is correct: I am 1.86m (or 6ft 1in) and did this on a road bike, mostly in the drops, with no aero gear or tribars). There was very little wind, at sea level and the temp was 71F.

The analyticcycling.com calculator tells me my speed should have been 11.96m/s, roughly 27mph, compared to the actual 22.4mph achieved. Time wise, that is 5min difference. What's happened to those 5min? To get to the actual time, without playing with other variables, I need to assume a CDA of 9.9 - does that look right? (Though tall at 6' 1", my BMI is 22.0).

Cheers for any thoughts