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Must you look at the little diamond-shaped arrow on the fuel gauge EVERY time you fill up to know which side holds the fuel filler? Have you ever pulled to the fuel island to discover you’re on the wrong side? Did you utter bad words before or after you said, “Why don’t they put fuel doors on the same side of every car?!?”
The answer to that question is complicated, if not convoluted.
Based on my research into the Federal Motor Vehicle Safety Standards, I came to the conclusion (a conclusion later supported by my contacts at both the Ford Motor Company and Nissan North America) that no U.S. government regulation concerns which side on which the fuel door must be positioned. Much to the chagrin of many motorists, the fuel door can be on either side.
With no legal or marketing motivation, and scant ownership enjoyment implications, car-company engineers are free to place fuel doors on whichever side offers the easiest packaging, according to Ford spokesman Mark Schirmer. He added that there’s not enough room — and no demand — for dual fuel doors.
Americans prefer left-mounted fuel doors, said Schirmer, referencing a Ford study. A driver’s-side fuel door makes it easier for drivers to place the car’s left fender close to fuel pump. Still, fuel door location is typically not part of the buying decision, added Schirmer.
Those in Japan, India, the United Kingdom, Australia, Hong Kong, New Zealand, and countries in southern Africa drive on the left side of the road and sit on the right side of the car, and it appears they prefer right-mounted fuel doors, given the tendencies of cra manufacturers. For at least 25 years, the conventional wisdom among auto writers has been that Europeans like right-side doors. However, when I posed this to my industry co-horts, no car company would speculate if or why that might be true.
Nissan, like most automakers, produces some vehicles with left fuel doors and some with right doors.
“The placement of the fuel door is mainly a factor of fuel tank design, location and underbody packaging,” Nissan’s Steve Yaeger wrote in an email. “With all of the structure and components located underneath the vehicle, (engineers) would quickly encounter restrictions in trying to route the filler tube to the same side on every vehicle.”
If mechanisms such as a “big, honkin’ speaker” must be placed on the left side, engineers put the fuel door on the right, notes Schirmer.
The bottom line: Fuel door position is not a random choice, but if engineers have a good reason to place fuel doors on the right, that’s where they go.
If you can’t remember the location of your fuel door, don’t be ashamed to look at the little diamond-arrow on your fuel gauge … BEFORE you pull up to the pump.
They follow precedent from other models in the company, but there is no strong tendency for one side or another overall. As a rough rule, Japanese cars have the filler on the left, German cars on the right, and American cars on the right but with weaker patterns. Why? An even distribution may optimize for gas station parking situations, depending on design.
Patterns: For single exhaust vehicles, the gas filling opening is on the opposite side of the exhaust, I've heard that this makes engineering the underbody easier: Japanese cars tend to have the exhaust on the right and American+German cars tend to have the exhaust on the left. Possible explanations for placing the filler opposite the driver include emergency fueling while pulled over. This also holds for placing the filler on the left for Japanese cars since Japan drives on the left 
Why the distribution may approach 50/50: Different engineering standards, and efficiency at gas stations: If everyone entered gas pumps the same way it would make conflicts and situations requiring backing in more common. In the worst-case scenario, only half the spots would be easily accessible. Simply pulling forward doesn't solve any problems, since drivers arrive and leave at different times, so having two 'types' of cars allows each type to find an ideal spot better. 
Japanese (90%+ left): Japanese cars are consistent within their model lines and even parent companies. The rate of exceptions is probably under 10% of vehicles produced in a given time period.
Nissan: Left (right: 350Z/G35 and 370Z/G37)
Mazda: Left (Mazda3 right due to Ford influence)
German: German automarkers tend to place the filler on the right, more consistently than American cars.
American: My knowledge of American cars is less, but they tend to be less consistent within their model lines and as a whole. Also, the relationship between parent companies is murkier.
Ford: Right (new Mustang is left, old Mustangs are right, Probe is left probably due to Mazda influence)
Chevrolet cars: Left.
Chevrolet Trucks/SUVs: Right
Dodge: Left (Viper is right)
Exceptions may include when automakers share platforms: Ford and Mazda shared many (Probe/MX-6, Mazda3/Focus, etc.)
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