What is the fuel cost of riding a bicycle?
Before you answer zero, consider the following: Riding a bicycle burns calories. Calories come from the foods that one consumes. Food isn't free. How ...
Before you answer zero, consider the following: Riding a bicycle burns calories. Calories come from the foods that one consumes. Food isn't free. How many additional calories would one burn per mile vs sitting in a motor vehicle for the same amount of time? What foods would provide the most calories per dollar spent? Could a person reasonably expect to maintain a level of physical health necessary to efficiently operate a human-powered vehicle on such a diet? What would be the nutritional profile of a diet that would support such an active lifestyle? What would be the cost per calorie for the healthy diet? Would regular exercise affect basal metabolism thereby increasing total caloric needs even when not actively riding? How much money would be needed to purchase one day's worth of the cheapest food to sustainably support a sedentary lifestyle? How much would a single day's supply of optimum diet cost?
I won’t address the minimum cost of a subsistence diet, because you could theoretically forage with the only cost being the time involved. Bums do it every day.
Incremental fuel cost of bike commuting vs couching is going to be very small. Again, theoretically, you are meeting your basic dietary requirements with your normal sedentary diet, so the additional calories required would best be met by carbohydrate fuels, like bread or pasta, which are not expensive. Fuel for a 15 mile commute would be perhaps 1000 calories for the round trip. The frugal cyclist could certainly come up with 1000 calories for less than $1 (think of something like a couple PB&J sammitches), while the same 30 mile round trip in a car would require at least 1 gallon of gas, at whatever the prevailing rate is.
Now if you chose to use premium fuel for your cycling, say something like Powerbars, then you COULD make your fuel cost for cycling higher than driving.
One of the savings generated by bike commuting that many people forget about is TIME. If you consider working out every day a necessity, as I do, bike commuting is a huge time saver.
Consider our theoretical 15 mile (30 mile round trip) commute. If it takes 1 hour by bike (reasonable) and 30 minutes in a car (reasonable, maybe even generous, given rush hour traffic), you get two one hour workouts for the "time price" of one, since you’d kill an hour in the car and still have to get your workout in.
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Edit: Changes in metabolism would favor the bike, if anything. Your calorie requirements drop back to sedentary level almost immediately after getting off the bike. Training effect over time would make your cycling more efficient, reducing calorie requirements slightly. "Metabolism boosting" post-exercise has been shown to be a myth.
How about this: By riding a bike you will be on average more healthy. Your pulse rate will be lower and you will be leaner meaning that you carry around less weight. All this means that the energy cost in resting state (maintaining body temp, breathing, heart function, moving your body mass around) are less than a more unfit person. I bet that in the end, you spend more energy while riding the bike, but you spend less during the day just for "staying alive".