Nothing wrong with expectations

But being that you haven't 'really said' you have, or noticed, any running/power/performance problems other than a lower MPG narrows the possibilities 'some'.. Also, you never indicated if the lower MPG was a gradual loss over time, or more rapid loss that might point to a hardware problem. Or both..

Has the compression ever been checked? Exhaust for Cat restriction for a 190K engine? Timing/advance per spec.? Fuel press? Or a tail pipe test reading may be of a benifit for a fuel/combustion/Cat efficiency for a general 'running condition' or done during the last smog ect..If everything proves out,and if you haven't noticed and performance problems and you've changed out all the tune up goodies, fuel delivery is good then you may get some results/benifits with the addition of a air inake, programmer to boost up the HP a bit.. But the flip side of the coin is that these add ons can cover the cost of a 'few' gallons of gas/fuel..

My 05/6.0 is running a flash tuner program and HP is stepped up to about 400HP from 325.. The beauty is,, that not only do the extra ponies come in handy, but highway MPG is increased slightly, or a wash if I can't keep my foot off the carpet..

So, didn't mean to rain on your parade, but before the addition of any 'pieces' make sure that the tune is spec, the engine can breath, and fuel is availableto go for a higher MPG bench mark....

 

2wd F150s had either 2.73, 3.08 or 3.55 rear. I think 4x4s only had 3.08 or 3.55 rear. In town 2.73 is the worst usually getting around 13. 3.55 is the best ratio for in town and people with that ratio usually get about 16-17. For highway mileage in your truck your doing about as good as you can with what you have.