F150 Man 4Eva, what I think is happening to you on the highway is that the air resistance might be getting you. I'm going to use the metric tire sizes (235/70-17) as an example because the first number is the width between the sidewalls, I'll also use the sidewall ratio of 70 to find height. A 30 inch tire is usually around 235 mm wide but a 35 inch tire is about 345. Since the width has to increase with height then by going up 5 inches on total diameter then you add 11 cm or close to 4.5 inches in width. If you hold your hand out of the window going 70 you'll see how much wind resistance just a small area must overcome.
I'm also not a firm believer anymore that adding small amounts of height (like a 2.5 inch leveling kit) causes as much drag as I hear on other forums. I think the mpg drain comes from the bigger tires that come with the lift for most people. I added a leveling kit but my tires still had a few miles left so I didn't upgrade them. I still get about 17-18 mpg going 85 down the highway(average from gryphon before and after the lift)...and my truck isn't exactly what people call aerodynamic. Just the CB antenna alone causes enough drag to notice in a heavy head/crosswind.
The tow tune is designed to improve performance under heavy load. From the engine's point of view load is anything that keeps the truck slower than what the driver wants it to be. Watch the load next time your using cruise control on the road. When you go down a hill you'll see the load go down, when you go back up the hill the load will go up. Adding drag is the same thing as adding weight from the engine's point of view, it still has to work harder for the same results. When the engine works harder it uses more fuel. I'm not sure why the canned tow would do better than the custom performance though. Does the custom get better mileage in the city though?
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