Help: Gas Leak?? Need Advice

Hi everyone,

I have been driving my grandpas old ford ranger because it had just been sitting in his yard for like a year, so I am borrowing it while I save up some money, anyways it used to be my sister's truck and when she had it, it apparently had a hole half way up the gas tank, (my gpa says he took it out and "saudered it" but I don't know if that really worked...) and the gas gage is apparently logged down, so it shows lower than it really is?? And it kind of seems to move around..    But my gas expenses are more than they should be, SO I dont know what to do because it is not my truck so I don't want to put bunch of money into it, but i dont wanna waste a bunch of gas,   How bad is it for a fuel injected truck to run out of gas?? My sister told me to run it on empty and track the mileage until I run out because the gas guage lies and says you're on empty when you are not.  So that I can avoid filling it up to where the guage says half but is really more than half and is leaking out!   And then to just keep track of the mileage, and stay under half a tank????   
Help Please!!
Thanks!  :)