Waldo, when you go around with your tailgate down, it is not the same as removing the the bed cover. Ford, in their information, recommends a bed cover for slightly better mileage, but says leaving your tailgate open will not get the same effect.
If you want to buy a louvered tailgate, do so, but I don't think you will achieve better mileage if you do. It is your truck and your call, but you asked our opinion and/or insight. Take it all with a grain of salt and make the decision with which you are happy.
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