Re: Actual size of the gas tank??
14.5 US Gallons is the volume of just the gas tank. But your gas tank has a pipe of about 1.25" diameter leading from your gas door to the gas tank. Filling up that pipe will let you hold extra gas, but it's not adviced. Reason for this is because that pipe just slides into your tank with just a rubber grommet keeping it "snug".
That grommet won't always be nice and tight over time and allowing gas to sit in that pipe will increase the rate at which that grommet ages. So one day that gas in the pipe will be wasted money because it will start to leak out thru it when your tank is overfilled.
Also, all gas needles fluctuate. They are just an approximation. And I believe, may be wrong, but I believe all US sold vehicles must have their gas light come on when there is atleast approximately 50 miles of gas left in the vehicle. Your computer is told what it's peak mpg is and figures, based on a varying gas gauge, when you have 50 miles worth of gas left.