Why am i not getting good MPG?

[F150 Man 4Eva]

Hey I appreciate your help and explaination. My main question was about the MPG differences between canned tow and custom 87 perf. City driving seems to be about the same. All is good on the custom 87 perf. until I get about 60 or above then it really drops. This is watching Instant Economy. I noticed two big changes from the tow tune to the perf. My K&N is so much quieter. That drone is all but gone. The other thing is the TC locking up. Before when I would hit about 45mph in 4th, the TC would lock. Now I can get up to 60 and it won't lock. I pretty much have to let off of the gas for a second, let it lock, and then resume speed. I don't want to come across like I am complaining, because I am not. I love the way my truck and tranny are performing. Its just that when I was getting 15 MPG highway, I was excited. I wasnt getting that before the Gryphon. But now with the custom perf..I am way below that. So I don't know what to think