I am getting 4 less miles per gallon than I have previously

I have a 2004 mustang, 6 cyl, with 61512 miles. A few months ago my fuel mileage average dropped from +25 mpg to +21 mpg. Today, I had a mechanic run a diagnostic but found nothing, no codes. The car runs and idles fine. I fill at the same station and rec hopefully, my message is enough for you to steer me to the next steps to take