Growing up all those years ago, my dad used to always say that the more air there is in the fuel tank, the more condensation can form in the tank. He always insisted that a fuel tank should never go below half. He said the condensation can clog up fuel filters, cause problems for fuel injection systems, and introduce corrosion in places it should not be. Also, while it is not that big of a deal in the summer time, in the winter, it can result frozen fuel lines. I have also heard that if you make a habit of running your tank too low on today's vehicles, you can damage the in-tank fuel pump. Now I personally do not know that any of this is absolute fact, but it all makes sense to me, so I try to always keep my tank above half, just to be on the safe side.