A person riding a bicycle at low to medium speeds (10–15 mph) has a highly efficient use of energy. Legs are most efficient at a narrow range of cadences: plodding up hills without switching to an easier gear or riding with legs spinning at a fast speed both waste energy, similarly to the way that a car gets the best fuel use in the proper gear. On a flat paved road a 150 lb. person uses approximately 30 watts of power to walk at 3 miles per hour. If that person were on a bicycle using the same 30 watts of power he would travel at approximately 9 mph, 300% more efficient than walking.
The drag created by air increases roughly as a square of speed, thus creating a speed barrier that even the most experienced cyclists cannot exceed for long periods of time. Amateur racers weighing 150 lbs are able to use 200 watts for over an hour, and a professional racer may be able to use over 1500 watts in the burst to the finish line, but this would be unsustainable for the course of the race. The limitations of sustainable power and aerodynamic drag are the reason that cyclists competing in races like the Tour de France ride in a pack or pace line.
Oxygen use during exercise is directly related to the energy being used, similarly to combustion. For experienced cyclists, a linear relationship exists for oxygen consumption at medium cycling speeds but at faster speeds oxygen consumption rises, making cycling less economical. The following chart compares the calories per hour used for walking, running and cycling at different speeds
We commonly refer to food intake in calories, but in fact calories are a measurement of heat or energy used. Dr. Edward Coyle of The University of Texas in Austin studied the average values of oxygen consumption by cyclists at difference speeds. As we ride faster we consume disproportionately more oxygen. At speeds exceeding 15 mph our overall cycling efficiency declines.
The force required to overcome air drag rises as the square of the speed. Though it is not a huge factor at low speeds, air resistance becomes the primary limiting factor when cycling at high speeds.