Why Do Cars Have Fuel Doors on Different Sides?

man pumping gas; via Flickr

Editor’s Note: Robert Frank, author of The Economic Naturalist’s Field Guide: Common Sense Principles for Troubled Times, recently guest-blogged for several weeks here at the Business Desk with answers to some of life’s economic ironies: why brides buy dresses while grooms rent tuxes, why drive-up ATMs have Braille dots, and why your fridge has a light but not your freezer. We asked readers for any economic ironies of their own that they’d like to pose to Prof. Frank. Today and tomorrow, we’ll post his answers.

Question: Why do some cars have the fuel filler door on the driver’s side while others have it on the passenger’s side?”

Robert Frank: This is actually one of the examples included in my 2007 book, The Economic Naturalist. Here’s the answer suggested by my former student Patty Yu:

“One of the most frustrating experiences of driving a rental car is to pull up at a gas pump as you would when driving your own car, only to discover that the gas tank is located on the side of the car away from the pump. Auto manufacturers could eliminate this difficulty simply by putting fuel filler doors always on the same side of the car. Why don’t they?

In the United States and other countries in which motorists drive on the right side of the road, it is easier to turn right than to turn left across oncoming traffic. A majority of drivers will thus buy gas at stations they can enter by turning right. Suppose gas tanks were always on the driver’s side of the car. Drivers would then have to park on the right side of an open pump in order to fill their tanks. During crowded hours, all spots on the right sides of pumps would be filled even while most spots on the left sides of pumps remained empty.

Putting fuel filler doors on different sides of different cars thus means that some cars can access pumps from the left. And this makes it less likely that drivers will have to wait in line for gas. That benefit greatly outweighs the cost of occasionally pulling up to the wrong side of the pump in a rental car.”

After my book was published, many readers wrote to complain that no manufacturers deliberately chose the locations of their fuel filler doors for the reasons suggested by Ms. Yu. Perhaps not, but that doesn’t mean her explanation fails. As in Darwinian evolution by natural selection, new patterns tend to persist not because they were consciously designed to be useful, but rather because they happened to serve a useful purpose. If the filler doors weren’t on different sides, there would definitely be more crowding at gas stations, for the reason Ms. Yu described. In that case, a car manufacturer who offered a car with its filler door on the side opposite most other cars would have a strong selling point. (“Buy the Chevy Lefty and avoid long gas lines!”) Ms. Yu’s observation explains why the current distribution of filler door placements, no matter how it originated, is an equilibrium.

Paul Solman adds: Bob has deftly slipped in one of the key concepts of economics here, and I think it’s worth noting. Equilibrium is that relatively stable situation to which there’s pressure to return whenever any individual deviates from it. So if I stand up at a NE Patriots game and everyone else is sitting, I won’t be left standing for long. If I sit when everyone else is standing, I don’t see a thing and had better stand up. Both extremes are equilibria; the pressure is to conform (though, as you can see, equilibrium is not necessarily forever).

When it comes to economics, equilibrium is famously that unit price at which supply and demand meet. If a supplier tries to charge more than the going rate for something, she’ll soon be brought back into line. Same for a customer who tries to pay less. She’ll pay up, or not get what she wants.

In the Case of the Ambidextrous Gas Tanks, “equilibrium” means the situation in which both sides get satisfied some, since all-right or all-left would create pressure for at least some gas tanks on the other side, as Bob suggests.

The reason I bring all this up is to repeat a key insight of Bob’s that I’ve summarized here before: his notion that all organisms, from bacteria to bonobos to bobos in paradise, have evolved an equilibrium with respect to the competing impulses to cooperate and compete. See this piece or, better, get a copy of his classic, Passions Within Reason.