According to an article from Time magazine, there is a better day of the week/time of day to purchase fuel. Fuel mileage consumers who tend to do more driving than the national average always say to fill up in the morning due to the lower temperature of the fuel. Check out this link and find out how to put more money in your pocket at the pump!
http://business.time.com/2012/08/29/why-wednesday-morning-is-the-best-time-to-buy-gas/