Horse Equivalents

What if we visualized cars with horses in front of them? We’ve gotten used to thinking about engines in terms of horsepower, but rarely step back to consider what that unit of power means: a rough equivalent in horses.

Consider the Geo Metro. It was one of the smallest cars available on the U.S. market in the last few decades. And yet its initial release was rated at 55 horsepower, and later models had 70 horsepower. That is, to move the relatively-tiny chunk of metal that made up that vehicle, with the acceleration that its engine could produce, would have required the equivalent of 55 to 70 draft horses pulling as hard as they could. Imagine the horses pulling the car: with the horses paired up, and allowing about 10 feet nose to nose for the horses, they’d make a train 270 to 350 feet in length, over 20 times the length of the Geo Metro itself. Do a similar calculation for all the other vehicles in the streets and the horses add up fast.

Why do such a calculation at all? It seemed to me that we’ve gotten used to the power of the machinery around us and have forgotten the scale. Even a lowly kitchen appliance like the blender can require an incredible amount of power: higher end blenders draw over one horsepower (i.e., more power than a large draft horse can produce), which means they require over ten times what a person can produce consistently if the blending were to be done by hand.

Using this horse analogy, we can produce a rough calculation of the amount of land it would take to harness the biocapacity of the earth for our global transportation system. Let’s take a conservative estimate and say that the average vehicle, including all cars and trucks, is 100 horsepower. Let’s use a conservative estimate of the number of vehicles globally—800 million. That’s 80 billion horses. Then let’s take a conservative estimate of the amount of pasture land required to feed a horse—1 acre. That’s 80 billion acres, which is 125 million square miles. (Let’s assume that the horse manure is somehow collected and used to fertilize the pasture.) Given that globally there are about 20 million square miles of agricultural land in total, that means we would need six times all the land available globally for agriculture just to feed the horses to provide equivalent transportation power.

Of course not all cars are in use all the time, so let’s be generous and assume that we don’t need quite as many horses (maybe they are shared between vehicles), and we cut it down by a factor of six. We’re still left with needing all the agricultural land worldwide just to power our vehicles. Worse still, using Odum’s observation that photosynthesis is likely the highest net emergy possible, using photosynthesis is the best way to sustainably provide such a power source, so alternatives are unlikely to be better.

Maybe there are some invalid assumptions I’m making here, and I’d love to hear about them, but the overall point is that in trying to reform our transportation system we can’t just expect to keep the same number of vehicles and replace them with more “efficient” ones, nor can we continue to build vehicles of the same power. We need far fewer, less powerful, and more efficient vehicles. So these days whenever I see someone talking about a “more efficient” car I wonder to myself “how many horses does it take to pull it?”

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Responses to “Horse Equivalents”

  1. horse power in a car or any sort of machinery is totally different to a horse .eg it would take 2 race horses to pull a small car to 40 mph why do i know this iv tried it
    so before you start saying it would take 70 horses to pull a car with equal horsepower than think again . ps shire horses are double the strenght of a normal horse but because of body mass they are slower . come on do you really think a mini would be able to pull a huge train i very much doubt it .