I have a 04 5.4L screw FX4 with 35 toyo MT's, stock 373 gears and a 6 inch Pro comp lift. I have installed a airraid intake; Banks Monster ex; Superchip reprogramer; bed cover; tb spacer every thing I can think of inorder to improve gas mileage. I drive 75 to 80 mph and am getting around 12 mpg. My next big idea was to regear to 456 to bring it close to stock settings but have been told with my set up and the speeds that I drive it would not change a thing. Should I save my money.

 

Well I have the somewhat same setup as you except for the bed cover, tb spacer and I have an 8" lift. I run the 4.56 gears. The most increase was in city driving. I got a noticable improvement. On the highway though I drive quite fast usually in the 85-90 range and I get about 12 or so but it holds the gears much better if you hit a grade and need more power.

I have not really calculated it too throughly yet. I do know it roughly takes a 1/4 tank of gas to go 100 miles with a up and down highway.

To me the gears were way worth it as it made the tunes come alive and the truck much more responsive in acceleration.

 

With a 6" lift, Edge and 4.56's I get about 14 in town, I have the 4.6 but I don't think it would make that much of a diffrence...might actually hurt?

 

Well the gears will help a LOT around town, however they'll end up hurting you on the highway compared to stock gears. Since, your running bigger tires it actually makes your 'gears' higher which would let you get better gas mileage on the highway but kill your city mileage and vice-versa with a lower set of gears like 4.56s. Its a trade off, personally I'd do the 4.56s and never look back.

 

Save your money.
Honestly the cost of gas although high as it is, is not easy to cut down via mods. If you save 1mpg that is 27 miles for me. 27 miles of gas is is $5. How long would it take for a mod to pay itself off at that rate? Not even to mention the loss of performance.
Re-gearing will not help your milage as much as you want it to. It just wont.

 

It is true that it would take alot of gas to make up for the new gears, however..... I put 4:56's in my truck and I must say it is like a whole new ballgame. Like the difference in a 4 cyl mustang and a GT if you get what im saying. You would be much happier with your truck and I think you will gain some of your mileage back.