Not sure how to ID what a "top tier" gas is. I analyzed my gas since I bought the car 8.5 years ago. No real pattern emerged. I detect no seasonal variation by brand and time of year. The mileages above 19.x involved long trips. The red highlights I either did not record the brand or my daughter/wife didn't write down the proper information.

Here's a thought...I have 147,000 miles on the vehicle ('02 3.0i) and changed the spark plugs once at about 92K. Might that contribute to lower mileage?

No seasonal variation detected
Brand/MPG (average)
Love's*18.47Express*18.17Liberty*19.10Gulf*19.10Shell19.14Sheetz19.41Unknown19.50Wawa*19.60Exxon19.67Costco19.737-Eleven*19.90BP*19.94Citgo*19.94Circle-K*19.94Chevron*19.97Hess*19.97Sunoco20.01Rutters*21.09Getgo*20.71Royal Farms*22.78Mobil*22.78* Statistically insignificant (5 or fewer tanks)