I am sure that you have heard it before, “Buy your gas in the early morning before it gets hot in the afternoon and you will get more gas for the same price”!
The theory being that gasoline expands when heated, thus taking up more volume in a confined space. Therefore, it seems logical that gas in the early morning would be cool and condensed into the confined space (your gas tank), while in the hot afternoon the gas would be expanded into the confined space.
This is indeed a sound theory. In fact, the gasoline in your tank is more than likely expanding and condensing in correlation to the temperature your gas tank is exposed to (like a hot black top highway, or cold snow and ice).
This is why the manufacturer always puts a message somewhere on the cap or near it stating to not “Overfill or to the top of the fill tube”. It does pose a danger. If you filled your gas tank so that the gas is level with the top of the fill tube, then only go a couple of miles and park in the hot sun, the gas could potentially expand right out of the tank (if a non pressurized system) or create a leak from expansion in the tank or tube (in a pressurized system).
However, that all being said, gasoline at filling stations are primarily stored in underground tanks surrounded by several inches of cement, then there should be a secondary cement wall encasement in case the first containment fails. So, needless to say, the gasoline is pretty well insulated from differing above ground air temperature.
In fact, ConsumerReports.org did an independent test at varying times of different days. The test measured the temperature of the gasoline coming out of the gas spigot. The result is that regardless of the difference in temperature in the outside surface air, the gas remained a constant 62 degrees (which is about the typical ground temperature). It makes sense, right?
It makes so much sense to me that I do not expect to see it any time soon on an episode of Mythbusters.
The bottom line is because the gas is stored in insulated underground tanks, it does not make a difference what time of day or what temperature it is when you purchase gasoline.
So, just another thing to remove from an already cluttered and worried mind, I speak of myself, not you.