Gas mileage and the temperature?

[tggroo7]

Alright, I'm not an expert whatsoever on cars, so that should explain why this sounds stupid if it is. First of all, cars get better gas mileage in the summer than in the winter, right? I started thinking about this as my current tank of gas is seeming to be the best tank I've had since hypermiling. I wondered why it is so much better than the last few, especially since it includes running late for work one day and throwing the hypermiling out the window (driving 70 in my detour of a long and slow train) for a few miles. So I wondered if this being the hottest week of the summer had anything to do with it (around 90 degrees for several days now). So, does it?

Also, what about the temperature exactly causes the better FE? Does it do something to the gasoline or the engine or what? Or is what I'm saying not even true to begin with (that FE is better when it's hotter). If temp does improve it, is it not possible to USE this fact? Obviously HEATing the GAS tank sounds like a pretty bad idea, but you'd think with modern technology, the car companies might be able to come up with some sort of safe way to do this. Or does it have nothing to do with the gasoline and maybe it just deals with the engine when starting up.

Keep in mind I know next to nothing about cars so forgive me if this is all sounds retarded.