I've upped my riding amount significantly in the past year or so and as a result am in much better shape. I use a Polar HRM that estimates calories burned from heart rate, and it has been giving lower and lower readings for the same ride and same pace, because my heart rate is lower. I also use a program that estimates calories burned based on speed, drag, climbing, etc, which I believe to be just calculating wattage output and deriving calories from that. Now it makes sense that wattage would be a much better indicator of calories burned (energy burned -> power out), but how much does your body's efficiency play a role in this? It seems plausible that if I rode with someone untrained for biking at 100W for an hour, I would burn less calories because I'm better tuned to the activity. Basically: am I necessarily burning the same calories by doing a given route at the same speed, even as this becomes less of a perceived effort for me to do so?