"The bicycle is the most energy-efficient mode of land transportation that exists.* Cycling burns about 35 kilocalories of food energy per mile [about 91 kJ/km], whereas walking the same distance burns three times as much. By comparison, car travel uses about 1,860 calories of fossil fuel energy per mile [about 4854 kJ/km]."
- Ian Roberts with Phil Edwards, The Energy Glut: Climate Change
and the Politics of Fatness (Zed: London, 2010), 103.
*A little web-searching suggested that ice skating may have a similar level of efficiency, though precise numbers depend on average speeds, body sizes and the quality of equipment in each case.