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With big wheels the best thing you can do for milage is make sure your wheel alignment is spot on and keep a consistent speed on the highway.
16 isn't too bad for a truck like that so I am guessing that you are taking care of it pretty well.
For the high topper I would think about building up the top of the cab starting at the front following the line of the windshield, sort of like an Xterra. If you used foam construction and went all the way back to the topper it could also help keep the cab a bit cooler in the sun.
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