GasBuddy analyzed three years worth of fuel price data and found that Monday offers the lowest average, making it the best day to fill up. Thursday is the worst day to buy gas, having the most expensive average.
To put the amount of savings into perspective: if every U.S. motorist bought gasoline on Thursday for an entire year, they’d collectively spend an extra $1.1 billion versus filling up on Monday – the lowest priced day of the week.
Source: GasBuddy