My first 5th wheel was towed with a 6 Cyl. Ranger and I felt everything that caused drag or power requirements. I surely felt the difference with or without a grilled Tailgate. I even felt the difference of the distance the trailer was from the cab, the closer the better. But with the diesels with more torque it is hardly noticeable. I can think it affects mileage. My friend had a Canadian fibregass roof deflector and it was good for his 98 Dodge Cummins that had no power, he got great mileage also, but for me I did not like the room it took for storage and installation. I read somewhere that a rear deflector on the top of the trailer is best and no one actualy installs any. I had a Stationwagon with the rear deflector and it was great for mileage and kept the rear window clean. It is harder to pull a vacuum then pushing air, thats why the cars have a smooth rear deck design. I do loose mileage empty with my grilled tailgate, but I would not have a truck without my Grilled tailgate. I made 3 of them so far and they are strong and practical. My last one is made of 1 1/2" square tubing and heavy flat expanded metal and as heavy as the stock tailgate. The ones you buy are so flimsy that they are only usefull when closed. I can open mine and load a load of wood on top of it, specialy long lumber. I do my own support side bars to be strong for gate loads.