According to the fuel-savings app GasBuddy, Monday is the best day to save money on gas, while Friday is the worst day to fill up.
The reason Friday is the worst...gas station are trying to make some extra money on people going away for the weekend.
Monday is the cheapest, because in general, the beginning of the week is a slow time for gas stations.
For me...I fill up my tank sometime after it gets under 1/4....and when I have time. I usually don't wait till i'm at 5 miles to empty, but HAVE.
Have a good week!