Walking vs Driving a VW XL1 – A Thought Experiment

Allegedly, the Volkswagen XL1 gets up to 300mpg on diesel. I’d like to compare that against a best guess for the average mpg (energy equivalent) of a human. I’m doing this purely on total available energy, not any formal Gibbs free energy analysis. Mostly because I don’t have enough empirical energy on human energy conversion, so I’m going on the USDA 2k calorie daily recommendation.

I’ve heard, though never formally confirmed with an authority I trust, that a dietary calorie is actually a kilocalorie, which is 4.18 megajoules (MJ). So, that’s a general upper bound on the available energy in a human “fuel tank”, assuming we derive energy exclusively from food and we eat an average diet, calorically speaking.

Diesel fuel has an energy density of 45 MJ/kg. That means, at roughly 3kg/gal, 135MJ available in a gallon. Interesting that a gallon of diesel contains over 32 times more energy than a typical American human consumes in a day. If the XL1 can go 300mpg, that’s 2.22 miles/MJ or 450kJ/mi.

The average brisk walking speed is about 4mph. If I were to walk all damn day, breaking only to sleep and eating while I walk, I could theoretically walk 64 miles. Over that distance, I would have used 4.18 MJ, which works out to 15.3 miles/MJ, or 65kJ/mi.

Assuming all this math is right (which is likely dubious), the automobile still has a long way to catch up. But, then I’m not factoring in the well-to-wheels analysis for production and distribution of the fuel, nor a field-to-feet analysis for the production and distribution of the food.