Monday. The answer is Monday. But the real money-saving tip is not just the day — it is understanding why gas prices change and how to time your fill-ups to save a couple hundred dollars a year.

GasBuddy analyzed millions of fuel price reports over several years and found that Monday has the lowest average gas price in most US states. Friday and Saturday tend to be the highest. The difference is only about five to eight cents per gallon on average, which sounds small. But if you fill up a fifteen-gallon tank once a week, that is around sixty to seventy-five dollars a year. Not life-changing, but not nothing either.

Why Mondays

Gas stations adjust prices based on demand and competition. Weekends are high demand — people are running errands, taking trips, doing everything they put off during the week. By Monday morning, demand drops and stations that raised prices for the weekend start bringing them back down.

There is also a wholesale price effect. Gasoline futures trading happens during the week. Price changes at the wholesale level often take a day or two to reach retail pumps, and by Monday the adjustments from the previous week have settled.

State-by-State Variations

Monday is the cheapest day in about thirty states. But it varies:

• Tuesday: Best in a handful of states including parts of the Northeast.
• Wednesday: A decent second choice in most places.
• Thursday-Saturday: Almost always the most expensive days.
• Sunday: Usually mid-range. Better than Friday or Saturday.

The specific day matters less than this rule: avoid filling up on Friday and Saturday. If you can shift your fill-up to Monday or Tuesday, you will save money over time.

Other Gas-Saving Tips That Actually Work

Use an app: GasBuddy, Upside, or your grocery store’s fuel rewards program. Kroger, Safeway, and similar chains give you cents off per gallon based on grocery purchases. I have gotten forty cents off per gallon at Kroger just by buying groceries I was buying anyway.

Do not top off: When the pump clicks off, stop. Topping off forces gas into the vapor recovery system, which wastes fuel and can damage the emissions system. You are paying for gas that is not going in your tank.

Drive slower on the highway: Above about fifty-five miles per hour, fuel efficiency drops quickly. Every five mph over sixty is like paying an extra twenty cents per gallon. I tested this on a road trip — setting cruise control at sixty-five instead of seventy-five saved me about four miles per gallon.

Check your tire pressure: Under-inflated tires increase rolling resistance. The Department of Energy says properly inflated tires improve gas mileage by up to three percent. Check them once a month. The correct pressure is on a sticker inside the driver’s door, not on the tire itself.

Fill up on Monday. Avoid Friday and Saturday. Use grocery rewards. Stop topping off. Drive slower. Check your tires. That combination saves me somewhere around three to four hundred dollars a year. It is not exciting. But it works.