I ran into a a blurb in the paper about the thermal expansion of gasoline. Bottom line, the "gallon" is defined at 60 degrees F, and gasoline has a pretty large thermal expansion coefficient (for a liquid). Add it all up, and it comes to a lot of money spent on "inflated" fuel:
http://www.consumerwatchdog.org/resources/DPReportHotFuelUSAJune07.pdf
Here in LA about the only time it is as cool as 60 degrees is at night, most of the year it is hotter than that, at times, much hotter. This suggests that the best time to fill the car is around dawn, before the gas has had a chance to heat up. It isn't so much that the big tank in the ground gets hot, that is buried pretty deep and probably does not warm up significantly. Rather, the asphalt and top foot or so of dirt, which sit in the direct sun and heat up, in turn heat the gas moving from the tank to the pump. Possibly the pump itself, which may also be in the sun, heats the gas.
There are devices that can go on the pump to compensate for this effect. However, amazingly enough, they are only in common use at gas stations in cold climates. Everywhere else, it is "too expensive" for the gas stations. In other words, the gas station owners don't want to sell too much gas to us as a "gallon", but are perfectly happy to sell us less than a "gallon" when the temperature expansion works in their favor. Big surprise.
Admittedly this is probably only tens of dollars a year for the average driver. Still, I'd rather they be in my pocket than in big oil's. I'm going to make a point of filling up on the way to work, rather than on the way home.