Well, it's easier to link to work other people have done : )
"Driving vs. Walking: Cows, Climate Change, and Choice", Michael Cohen and Matthew Heberger, Pacific Institute
http://www.pacinst.org/topics/integrity_of_science/case_studies/driving_vs_walking.pdfThe Pacific Institute study compares
only the GHG emissions from the "fuel" energy inputs for walking, biking, driving a car. The summary on p.4 indicates that walking on a typical US diet generates about 25% of the GHG emissions as driving, and biking is about half that again. A vegan would generate about 20% of the GHG emissions as the typical US human-powered commuter.
A significant weakness in the PI study is that no consideration is given to the source of the food, which is a significant factor in the GHG emissions.
"Life Cycle Assessment of Transportation Options for Commuters", Shreya Dave, MIT
http://web.archive.org/web/20110715083534/http://www.pietzo.com/storage/downloads/Pietzo_LCAwhitepaper.pdfShreya Dave compares the GHG emissions for manufacture, disposal, and infrastructure required for use for walking, bikes, ebikes, automobiles, and various forms of public transit. Notably, she does not consider the GHG emissions from food production.
It has been assumed that, in the United States, a commuter will not change her eating habits if she
switches from driving to walking to work, given a reasonable walking distance and assuming sufficient
nutrition is available. (p.5) ... As with walking, food intake is assumed constant, and CO2 emissions are the additional CO2 release from the rider being at rest. (p.6)
Her summary, FWIW, is that walking, biking, and ebiking are responsible for the same amount of GHG emissions per mile. I haven't tried running numbers for an electric motorcycle, but it's probably about twice as emissive as an ebike.
Given that Gavin is gaining weight and thus presumably has not altered his diet downwards while riding the Enertia, Ms. Dave's assumption appears to hold true for him. In this case the Enertia likely is responsible for more GHG emission than his pedal bike.
In the eventual case where he reaches a steady weight and continues to bike, he will need to increase his diet to maintain his weight. Once the GHG emissions for his increased food consumption come into play, the pedal bike is probably no more efficient than the Enertia and possibly much less so.
