Originally Posted by cyccommute

It not that simple. The problem your analysis is assuming the same exertion. A mountain bike with knobby tires is harder to push along over a given distance than a road bike. Aerodynamics and rolling resistance are higher for the mountain bike than the road bike and thus mean that the mountain bike takes more energy to move over the same distance at the same speed as a road bike. You could increase the speed and effort on the road bike but then the exertion isn’t the same. And, if you increase the speed and effort on the mountain bike, it takes more energy to move it.