We have this guy at work who forwards on bad forwards, like the kind that should be checked at Snopes. Today, someone sent out a link to GasBuddy.com, which I can understand considering what fuel costs. Others added tips (that sometimes the prices on GasBuddy are for E85 or diesel, not regular), but this guy sends one that one should fill up when temperatures are cooler, as you'll get more gas due to thermal expansion/compression. I immediately think this is garbage, and start doing some research. The LA Times apparently had an article on this. Two interesting nuggets: there's a trucker who complaims about losing 3% volume due to temperature change, and a California study that talks about the low monthly average temp of the underground fuel storage tanks is 64 degrees, with the high being 83.
Now, the volumetric coefficient for gasoline is 9.5 * 10^-4. Assuming a termperature change of 19 degrees F, or 10.556 degress Celsius, we're looking at a change of just around 1%. To get a 3% volume change, we'd have to have a temperature change of just over 31 degrees Celsius or 56.84 degrees F. Let's say I'm dubious to say the least of that much of a temperature change. Realistically, it looks like one might save 1% by getting gas when the temperature is cooler. I figure I get about 10 gallons a week, so for a year 520 gallons, or at $3 a gallon, I could be saving $15.60 annually (assuming that I'd be able to fuel up when the gas is 10 degrees cooler).
With regards to GasBuddy, and finding the cheapest price per galloon, I get around 10 gallons when I fill up, so I might save $2 max if there's some huge price difference. At current prices, $2 is maybe 2/3 of a tank, or 20 miles optimistically in my Civic. In other words, if I have to drive more than 20 miles out of my way, the cost saving isn't worth it. Factor in time, and hunting for low gas prices seems to be a break even proposition to me, at best. Theoretically, if I could get my gas 20 cents cheaper per gallon, I'd be saving $104 annually. For arguments sake, let's say I value my time at $10 an hour, so for the 52 trips to the gas station I'd make, for me to really be saving money, it would have to be less than an extra 10.4 hours out of my year, or 12 minutes a week. I could easily see it taking me more than 12 minutes to get to a different gas station than the one I stop at on the way to work. In other words, the savings here are negligible at best. Of course, the economies here could change drastically if you use a lot more fuel, but for me, hunting for low gas prices just doesn't make sense.