I just returned from a drive from Chicagoland to Maine, and back.
US 90 most of both directions.
27 MPG average, even with the time through Chicago & that State of construction once called Ohio.
AC/defoggers on the whole time as it rained most of the time.
High test all the time, I have learned the performance and added mileage is worth it (plus 94 octane Sunoco on the turnpike in Ohio was $2.50 per gallon !!)