Why? Because (and here, we finally get to the real answer), 
  that's how wide a Roman chariot needed to be, in order to 
  accommodate the width of the war horses pulling it. So, in 
  some sense, the width of cars is not only an artifact of 
  the way that things had always been, but is also an 
  extension of the tyranny of Caesar. Remember that the next 
  time you go pick up some groceries.

http://www.snopes.com/history/american/gauge.asp

  This item is one that, although wrong in many of its details, 
  isn't completely false in an overall sense and is perhaps more 
  fairly labeled as "Partly true, but for trivial and unremarkable reasons."