I Like to Ride My Bicycle

There has been some talk on the web as to whether riding a bike uses more energy than driving a car. I decided to check the numbers, because my gut instinct was that it is way, way more efficient to move myself and a 20 pound bike to work, than it is to move myself and a 3000 pound pound car to work. I would have guessed, say, 100 times more efficient.

There are some analyses on the web, but they tend to miss things (i.e., where food comes from) or be unecessarily convoluted.

I start with our cars. A gallon of gas contains about 31,500 Calories per gallon. Our Prius is getting 40 mpg, which translates to 788 Calories per mile. My 1974 BMW gets at best 20 mpg, or 1575 Calories per mile. If I only drive it short distances, I get closer to 14 mpg, or 2250 Calories per mile. This has to be corrected for the amount of energy needed to extract and refine petroleum so one gets gas, but since roughly half of a barrel of oil can be converted into gas, I doubt the numbers different by more than a factor of two.

Riding a bicycle burns about 42 Calories per mile, if you're riding at a leisurely pace (10 mph). If I go faster, air resistance causes me to burn maybe 50% more. Some of this energy (roughly 8 calories) would be used anyway if I were just watching TV, to keep my brain warm. So, lets say 34 Calories per mile to ride a bike to work, instead of calling in sick. On the face of things, then, it would appear that cycling is at least 23 times more efficient that our Prius. However, food needs to be grown, packaged, and delivered. In the US, for an average diet, every Calorie you eat took about 10-15 Calories of energy to grow, package, distribute, and prepare (link). So, it really takes at least 340 Calories per mile to ride a bike.

Now, cycling has other advantages that one could incorporate. For instance, I will be more healthy. Although some seem to claim this is bad for the environment --- if I live longer, my energy-intensive lifestyle is prolonged --- I think this argument is specious (killing myself is NOT an acceptable solution to our environmental problems!). Of course, bicycles cost a lot less energy to manufacture, but since we insist on having both a car and a bicycle (my wife won't let me carry our daughter on my bike), that point seems a bit moot as well.

Therefore, riding a bike is only about 2 times more efficient than driving a single person in a Prius, and about 4-5 times more efficient than a more ordinary car (assuming my BMW is ordinary, which it is not, because it is one of the best cars ever made!).

Now, if I carpooled, I'd be using the same energy to drive as to cycle. A full Prius would be 2 times more efficient! A scooter, at 70 mpg, is equally efficient as a bicycle. Cycling isn't as green as I thought, because our system for producing and distributing food is inefficient! To really make a difference, I'd need to ride my bike AND buy my food from local sources.

As a corollary, I now understand why machines will end up ruling the earth, and why the premise of The Matrix is really dumb. Meatbags waste energy!

Comments

[alpinebutterfly]

Hmmm, but don't you have to buy food, even if you ride in / drive the Prius?

[Michael Muno]

I assumed that had I not done the exercise, I would not have had to eat as much. Of course, exercise is has its own benefits for health.
If I had an easy commute, and no child to bring to day care, I would ride my bike to work for the exercise. However, exercise is irrelevant to the environmental argument. . .

[Cindy]

Wouldn't the production of electricity be relevant in your calculations? I assume your car battery could be used for other things, like a paper weight.