What difference does a day make? When it comes to buying gas, not that much.

A study by Gasbuddy.com found that prices vary predictably day to day, but over the course of a year an average driver will only save $20 to $30 by gaming the system.

That said, there’s no reason to pass up the equivalent of a free fill-up, and according to USA Today, the best time to fill up on average is Monday morning. The worst? Fridays.

Gasbuddy.com says it’s all about good old fashioned 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," Patrick DeHaan, GasBuddy head of petroleum analysis told the newspaper. "We generally see more volatility and higher prices later in the week."

The ideal day does vary state to state, with Monday being the best in 29 and the District of Columbia and Tuesday in 19, while Wednesday is bargain day in Hawaii and Sunday in Utah.

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

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