That study by professor Ulrich of Wharton was based on the assumption that if a couch potato car commuter switched to cycle commuting they would get healthier and live longer. His analysis suggested they might save a little energy, but only a trivial amount in the big picture. But generally the switch to bike commuting, or going beyond that and living car free, involves more than just that one change, such as eventually moving closer to work, owning one less car and replacing it less often, etc. Furthermore, if the non-cyclist was keeping fit by going to the gym etc. before switching to bike commuting, then the longevity argument fails.

http://opim.wharton.upenn.edu/~ulric...viro-jul06.pdf