Gas Door on Driver's Side

Have the gas door on the Driver's side of the vehicle. It's safer because you can be right there to make sure that the pump doesn't overflow and the only unlocked door will be right where you're standing.
Andrew S 03/13/2012
Some Ford's have their fuel fillers on the driver's side. Some on the passenger side.

There is also a safety reason for having fillers on the passenger side. Imagine running out of gas on the side of a busy highway. If the filler is on the driver's side, you basically have to be standing in traffic to use a gas can. On the passenger side, you are much safer.