Fill your tank until pump clicks. Drive it. Then re-fill until it clicks. Divide the miles by the gallons. That is the true MPG. The trip 'computer' isn't always accurate. You stated you do a 'fair amount of sitting'. That kills your MPGs. If turbos are on boost, more fuel is burning. The motor is Eco or Boost, not both. Most engines should perform better on Premium. My 2011 Mustang 5.0l gets 18mpg+ and 2001 Lincoln DOHC 4.6l also gets 18mpg+. Both only run on only premium. My 5000lb Expedition gets 12mpg, no matter what fuel or how I drive it. Cleaning sensors or disconnecting the battery is a waste of time.