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Neil on Wheels: Gas Caps

Why isn't the gas cap in the same place on
every car?

Mazda Kiyora on the highway
Photo caption
A Mazda Kiyora speeds along the highway.

This is a fascinating question, one that involves both microeconomics and macroeconomics. On the micro side, you have the historical preferences of different carmakers. For example, the Porsche 911 has its fuel filler on the right front fender. Back in the 1960s, there was a logical reason—the gas tank was in the front of this rear-engined car—but now the fuel filler is there largely as a matter of tradition.

In sporty cars especially, designers like to position the weight of the fuel as close to the center of the car as possible (20 gallons of gasoline weighs about 120 pounds). BMW, which obsesses about weight balance, always puts the gas tank and the gas filler on the right rear of the car to counter the driver's weight on the left. Pretty cool, huh? Now, forget all that. Gas tanks can now be molded to fit almost anywhere, so it would seem that the fuel filler cap could follow suit. The Honda Fit, for example, places the gas tank under the front seat (it's perfectly safe there . . . I guess). Yet given the engineering demands on each car model on the road, it would be impossible to standardize fuel filler position. And maybe you wouldn't want to. Here's where macroeconomics comes in. If fuel fillers were all on the same side, wouldn't that mean that one side of the gas pumps would be busier than the other? Sure, you could pull up to the pumps from the other direction, but not all gas stations have good two-way flow. It has been suggested that the distribution of fuel fillers is an unintended consequence with desirable results.

In the end, the fuel-filler dilemma arises out of our own prosperity. If you own one car, you soon remember which side the filler cap is on. If you live in a multicar family, however, you can easily get confused. That's why fuel gauges now show an arrow or an image of a hose on the side you should present to the pump.

So before you start wandering around your car looking for the fuel door, check the gauge and be grateful for small favors.

This article was first published in May 2010. Some facts may have aged gracelessly. Please call ahead to verify information.