Hum I'm reading the thread and I'm asking myself, could it be your gas tank that actually leak. That is maybe why you aint getting good MPG, some of your gas just isnt consumed. It probably have many ways to find out, any advice on this guys?

Its just an option.

while I haven't had many leaky gas tanks in the past. Its something I would look into as well. You might also try running some marvel mystery oil through the engine and gas tank. While this wont fix the problem. It will ensure that things are clean.