I only get appx 20mpg at 70mph with little to no wind. Our octane ratings here are 85.5, 87,91.
Ok, this is my 1st 4.6 vehicle but I have customers with 4.6 Thunderbirds and 4.6 Towncars and we all get about 25-26 Hywy on 87 gas going 75 MPH.even with 4 adults and a trunk full of suitcases.
Again,I have limited experience but I still think I would check the usual suspects for the crappy mileage,ie. air filter,Mass Air dirty, low tire press.,worn plugs,etc.