OK, I'm still not convinced. Let me pose a slightly different scenario than the two rider one and show me the math so we can put this thread to bed. I ride today for 30 miles in 90 minutes. Tomorrow I ride the same route in 120 minutes. Show me how the amount of kcals used would be different in the two rides. Assume the rider is 160#. Also FYI: Joules can be converted to Watt-hours but not directly to watts. One is a measure of work the other power, not the same thing. Another thing: I'm looking at energy tables for running & cycling from McArdle & Katchs Exercise Physiology, Energy, Nutrition and Human Performance. Energy for a 71 kg man to run a 11min mile is ~10 kcals/min, to run a 5.5min mile uses ~20 kcals/min; hes running twice as fast and burning twice the kcals, not 4x the kcals as was postulated in one of the posts. The tables for cycling show the same increase in kcals based on increases in speed. Cycling at 5.5 mph uses 4.5 kcals/min, at 9.4 mph it is 7.1 kcals/min, 16 mph 12 kcals/min, looks fairly linear to me. Anyway, I look forward to your replys. Thanks.