Since I got my power meter, it is interesting to see how much energy (Kjoules) I produce on a ride. I always knew that most of the HR / speed devices were inaccurate and overstated the calories but what was slightly surprising was the amount. Generally, on a hard 3 hour ride I will burn approximately 500 calories an hour. However, that includes at least 4x10 or 2x20 z4 efforts and at least an equal amount time in z3 with an occasional z5/z6. There is zero energy used on descents no matter how fast one goes unless one has to pedal. On rolling terrain, power is easy to make while climbing but once you crest a roller, unless you shift into a bigger gear and really focus on keeping the power up, it drops quickly even though you are pedaling and going fast. To keep the power and energy up, one must go really fast down hill and pedal hard.

IMO, this is one area where the HR based speed calculations go wrong. After you crest a hill, your HR will stay elevated but your energy output is way down and you are now going fast. Eventually HR will catch up but it is just not that accurate. And of course there are aero differences between riders and riding position and wind.