This is a difficult question in that by engineering standards there is no limitations on where to put it other than space, form, and expense. By practical standards, this is a little more evident considering who might be buying the car. If you consider a driver side gas cap, which is what I think you are asking (not so much about tank placement), then it is easier for a driver to manuever next to a pump as most drivers in America (right side of the road drivers) are better at judging clearances on the left side of the car - this is easier for the driver to see and most drivers use the left side of the vehicle to maintain lanes and clearances. The problem with this, however is that it is inconvenient in that you are sometimes hindered by opening the car door inbetween you and the pump when you get out to fill the tank. The driver side gas cap is also easier to see in the rear view mirror when aligning the gas cap door close to the pump mozzle you will use. The passenger side gas cap has all the disadvantages of manuevering close to a pump, but you never have to worry about not being able to get out of your door because you are too close - unless of course your passenger wants to go in and buy a beef jerky.

Vehicle fuel cells are now typically placed for impact safety purposes, and to provide convenient access to the pump and filter. And the filler cap, the same applies, position the hose next to the driver, the person normally fuelng the vehicle.

I like mine on the driver side. My husband's is on the passenger side and it make dealing with the attendent doing the pumping a little harder.

I think they flip a coin.... really.

The placement of the filler is dictated by the placement of the tank. The placement of the tank i dictated by the space available and crash requirments. Most people prefer the filler on the drivers side so if everything is equal that is where it goes. Sometimes however the packaging ends up putting it on the other side. Also, cars are now designed for the world market and they may make a Left hand drive and a right hand drive version. So someone ends up with the tank on the non drivers side. ISO standards are placing a small arrow on the fuel gage that points to the side the filler is on. Look at your vehicle and you may find it there. I drive company vehicles at work and the arrow saves me a lot of hassles.