I've just read a bit by lifehacker which I found through jalopnik praising the little arrows which point toward your gas cap. Users offered several theories to help the arrowless motorist tell which side his tank is on:
"the tank if often on the same side of the car as the gage is of the instrument cluster"
and my favorite-
"The gas tank filler cap is almost always on the opposite side of the car of where the exhaust muffler is."
BUT- this is the real reason for gas cap locations:
Foreign gas stations don't have much room to drive around in, and gas stations in the US do.
So in the US, drivers can leave room between the car and the pump, open the door, and pump the gas. American cars typically have the gas cap on the left.
In cramped foreign gas stations, drivers sometimes have to park really close to the pump, get out on the other side, walk around, and pump the gas. So foreign cars have the gas cap on the HOME MARKET'S passenger side. Nobody switches their gas caps when they export cars.
So German, Italian, and French (all left-hand drive countries) cars have gas caps on the right side, passenger in the home market.
English, Australian, and Japanese (right drive countries), have caps on the left side, passenger side in the home market.
If a car has been badge-engineered, you must think where it originally came from.