17 July 08
Inspired by David Mackay’s online book Sustainable Energy — Without the Hot Air, I worked out just how much more energy-efficient a bicycle is than a car. Here’s the math: the energy content of gasoline is about 10 kWh/liter. Our car gets around 25 mpg (none-too-efficient, I know) which equals 10.6 km/liter. Dividing through our car therefore uses 0.94 kWh/km. As for the bicycle, at a modest pace of about 20 km/hr, I use about 15 kcal/km of energy (this value comes from the excellent book Bicycling Science, by David Gordon Wilson). At 857 kcal/kWh, I thus use 0.0175 kWh/km on my bike. The bicycle is therefore 0.94/0.0175 or about 50 times as efficient as the car.
In working out these energetics problems, one quickly runs into a plethora of conversion constants. Here’s one for fun: since a kilocalorie is the same as a food calorie, driving the car 1 km uses the energetic equivalent of a pint of ice cream.
We’re doing pretty well though in minimizing our car travels. Going through our expenses this evening, I worked out that over the past six months, we spent far more money on bike maintenance and parts than we did on gas (those bicycle overhauls add up).