Adding to what davidgcet said; the line on your 5 gal gas can is very, very, very approximate and is only usable when determining how much oil to add if you are mixing fuel for a 2 cycle engine. The Weights and Measures department that tests these pumps uses a device called a prover which has a custom designed narrow neck with a sight glass that after being properly calibrated is very accurate but pretty much useless as a portable gas can.
GTyankee also brings some great information to the table (Thanks for posting the temperature correction chart!
) that has an affect on how much fuel you purchased. In Canada your fuel purchases are corrected to 15deg Celsius and in the US to 60 deg Fahrenheit (Basically the same temperature)
What this means is the expansion of fuel with temperature is great enough to take into consideration. This also ensures the issues can be caught using the daily (or more often) tank dips which are also temperature corrected. If the dips do not agree with the pumps, then either the pump is wrong or the tank is leaking.
I have heard the claims that yo should buy your fuel early in the morning when it's cool to get the most fuel. Although this may have been true many years ago, the temperature correction system makes this no longer effective.
Another consideration; if the station has in-ground tanks, the temperature of the fuel is very stable, and remarkably close to 60 deg F. The time of greatest fluctuation is when the tanks are filled. On a hot day, the tanker truck is pouring in some pretty hot fuel that may take a day to stabilize. Conversely, in the winter (especially in the northern states and Canada) the fuel is pretty cold when poured into the tanks. These are the times the correction at the pump is the greatest.
In the end you will pay the same price for the amount of 'energy' you bought, which in the case of gasoline is measured in BTU.
The EVIC in your truck is a 'guideline' and will never pass a calibration test for a 'legal for trade' liquid meter. In fact, it is not capable of even getting close as it uses the gas gauge as an input device to know how much fuel is in the tank. I know that most people know their vehicle well enough to know if the top or bottom half is bigger. In calculating mpg accurately, the top and bottom halves MUST be exactly the same. Although our fuel gauges are much more accurate then they used to be, they are still just a general guideline.
Hope this helps.