I would probably be more at home putting this on the "Car Free" forum, but I'm not care free so tough. As an engineer, I was curious about bicycle efficiency, especially after reading some item on the net about how the bicycle is the most efficient transportation mode, even over walking. For a start, I figured I'd take a look at a bike vs. a compact car (something that gets about 30 mpg).
I checked some fitness websites, and most agree that a cyclist doing 15 mph is going to burn about 600 calories (actually 600 kilo-calories which is what your food is actually measured in) per hour. This is 2.508 megajoules / hour in standard units.
As an aside, I realize that if the cyclist is cranking at about 175 watts, this means that 25% of that 2.5 MJ per hour is reaching the cranks, but I figure that this is about right. After all, only about 25% of a 30 mpg car's gallon of gasoline is getting to the drive wheels too. I'm sure you other commuters realize the missing 75% of the energy is what is making you so sweaty.
Anyway, 2.508 million joules per 15 miles means a cyclist is consuming 167,000 joules per mile at 15 mph.
Looking at the car, it burns 0.5 gallons of gasoline containing 60.88 megajoules to go the same 15 miles. This is a simple 4.06 megajoules per mile.
In short, the car needs 24 times as much energy per mile as the cycle (this actually struck me as low.....if I put 5 people in the car it's only using 5X the energy as the cyclist per person delivered). On the other hand, 100% of the car's fuel is non-renewable while the cyclist runs on 100% biomass fuels.
Sorry to bore anybody, but I thought this was interesting and I figured I'd share my quick and dorky calculations