Some people say it's better to buy your gasoline first thing in the morning, rather than in the heat of the day. That's because gasoline, like all liquids, expands when heated. So, according to this advice, gasoline will be denser in the cool of early morning, meaning you'll get more energy per gallon than later in the day.
The basic facts are correct, but the advice is not. Gasoline does expand and contract a little depending on its temperature. When gasoline rises from 60 to 75 degrees F, for instance, it increases in volume by 1 percent while the energy content remains the same.
But filling stations typically store their gasoline in underground tanks, where the temperature variation during the day is much less than in the air above. The result is that the temperature of the gasoline coming out of the fuel nozzle varies very little, if at all, during any 24-hour stretch at any particular station.
Craig Eerkes