Everyone has a theory about when to buy gas. “Monday mornings, the stations reset their prices.” “Thursday afternoons before the weekend rush.” “Never on Friday.” My dad swore by Tuesday. I drove past the same three gas stations every day for a month and wrote down their prices.

Tuesday was not the cheapest day. Neither was Monday. Here is what the actual data said.

What the Data Showed

I tracked three stations — Shell, Costco, and an unbranded independent — for four weeks. The pattern was consistent across all three: prices dropped late Sunday night and stayed low through Monday morning. By Monday afternoon they started climbing. By Wednesday they had peaked. Friday and Saturday were the most expensive.

The cheapest time to fill up was Monday before 10 AM or late Sunday evening. The most expensive was Friday afternoon, right before the weekend. The difference per gallon was usually five to eight cents. On a fifteen gallon fill-up, that is about a dollar savings per tank. Over a year of weekly fill-ups, that is fifty dollars. Not life changing. But it is fifty dollars I would rather have than give to the gas station.

Why This Pattern Exists

Gas stations adjust prices based on demand. Weekend travel pushes demand up, so Friday prices rise in anticipation. Monday morning has the lowest demand — everyone filled up for the weekend already. Stations lower prices to attract the Monday commuters who are running on empty after their weekend trips.

The Bigger Savings: Where, Not When

Costco and warehouse clubs consistently beat the major brands by fifteen to twenty cents per gallon. That is two to three times the day-of-week savings. If you have a membership or live near a wholesale club, filling up there matters more than which day you go. The unbranded independent station was between Costco and Shell — cheaper than Shell, not as cheap as Costco.

Also: do not use premium unless your car requires it. The Federal Trade Commission explicitly states that premium gas provides no benefit for cars designed for regular. That is an instant twenty to forty cent per gallon savings if you have been buying premium unnecessarily.

Monday morning fill-ups saved me about fifty dollars a year. Switching from Shell to Costco saved me about two hundred. Together with dropping unnecessary premium, I am saving over three hundred dollars a year on gas without driving any less.