Want more gas for your money? Whenever possible, fill up when temperatures are relatively cooler. As temperature rise during the warmer times of the day, especially in the summer, gas expands and thus you get a little bit less as you fill up.
But in the morning or late evening, gas has more density and you'll thus get more for your money. Cooler temperatures also mean less evaporative emissions from your time at the pump — smog forms more easily when it's hottest outside.
Another good reason to buy gas in the early morning: It's speculated that gas stations, especially the corporate chains, adjust their prices in the late morning or at lunchtime. When those prices go up, that starts a domino effect for all the locally owned stations in the area.
John Rockhold is a green car enthusiast and Contributing Editor for MOTHER EARTH NEWS. Find him on Google+.