Why am i not getting good MPG?

[F150 Man 4Eva]

Its me again. I searched for miles per gallon and find this thread. I just had a few question about my gas mileage. Before I start, let me say that I know that my 35" Nitto Trail Grapplers that weigh like 80lbs. a piece are a BIG factor in my mpg loss. Here are the facts..which within lies my question. With my mods (K&N FIPK and Flowmaster 50 series) and my 35" tires, but before I ever got the Gryphon...my MPG would vary between 12-14. Now when I got the Gryphon but only had canned tunes...I ran the canned 87 Tow tune. Mileage around town was about the same, maybe a little better. On the interstate however (going like 75) I was getting around 15. This made me happy. Now maybe I was thinking wrong, please tell me if I was, but I assumed when I got my custom tunes and ran the custom 87perf. that I would get better MPGs. Well going around 50, I can get like 16-17, going on the interstate (70-75) I am getting right at 11. Whoa now. What happened? Is the custom perf. supposed to give the best gas mileage? Let me also throw in that I am NOT out to get 17-18,hell even 16 Mpg, and I know I have a big 4x4 truck not a car. And I also know it is pretty much give and take with power and MPG. I just need to know if this is normal. Thanks yall...Travis