From day to day the cost of gas varies predictably, but only about $20 to $30 per year can be saved over the course of a year by gaming the system.

According to USA Today, Monday morning is normally the best time to fill up your gas tank, while Fridays are the worst.

The reason? Supply and demand.

“Very early in the week, when gas stations are generally a little bit quieter traffic-wise, is a great time to fill up,” said Patrick DeHaan, GasBuddy head of petroleum analysis. “We generally see more volatility and higher prices later in the week.”

The best time of the week does vary state to state, with Monday being the best in 29 states and the District of Columbia and Tuesday in 19 states.

Here’s the full list for best days to hit the pump:

Monday: Alaska, Arizona, California, Colorado, Connecticut, District of Columbia, Florida, Idaho, Illinois, Indiana, Kentucky, Maine, Massachusetts, Michigan, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Dakota, Oregon, Rhode Island, South Carolina, South Dakota, Vermont, Virginia, Washington, West Virginia, Wyoming

Tuesday: Alabama, Arkansas, Delaware, Georgia, Iowa, Kansas, Louisiana, Maryland, Minnesota, Mississippi, Missouri, Nebraska, North Carolina, Ohio, Oklahoma, Pennsylvania, Tennessee, Texas, Wisconsin

Wednesday: Hawaii

Sunday: Utah