Liquid expands when it is warm, and shrinks when cold.
That is how a thermometer works.

Gas does this too.
So, why not get a little more gas for your money?

Buy gas early in the morning when it's cooler.
That cold gallon you bought will expand a little when it warms up.
Sure, the tanks are underground so the temp may not vary that much, but at these prices every little bit helps.

This may be another variable explaining our MPG discrepencies between the pump and the dash.

So we should only burn hot gasoline? :D Sorry I could not resist

Then again the size of the gas tank also changes with temp.

I found this physics101 thing about it.

http://216.239.57.104/search?q=cache:Rjuao...+gasoline&hl=en