Does it matter which day you fill up the car? A study by GasBuddy.com found 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. Still, USA Today reports the best time to fill up on average is Monday morning, and the worst day to do it is on a Friday. GasBuddy says that all comes down to supply and demand, as usually fewer people are getting gas on Mondays compared to Fridays. But even then, this isn’t a hard-fast rule, as it depends on where you live, though in most states Monday was the best day to fill up on the cheaper end. Read more from Fox News.