The next time you fill-up, take a look at some of the warning labels affixed to the gasoline pump. I saw the above label recently while purchasing gas from Costco.

This device dispenses gasoline solely by volume measured in standard gallons (231 cubic inches). It does not adjust for temperature or other factors which may affect the energy content of each gallon dispensed.

Makes sense. Obviously, if you purchase gasoline in the morning, each gallon of gasoline would presumably be denser because of the cooler temperature. I was going to research this issue, but Snopes had already beat me to it.