i have a 03 5.4 SC f150 with 3.55ls gears and i cant for the life of me clear 10mpg.... it doesnt matter how i drive(empty) 10 is the max i get, if i pull my 12ft vfront fish house i get 8.5 if im babying it. i have an airaid intake and a 3 inch SS exhaust kit... when i bought the truck i could get up to 15.5ish buyt 13.5 was the norm....

what am i looking at?

sensors?
plugs?
plug wires?
fuel filter...?

 

well i feel better as i get 13-14 and i drive it gentle ....i put on 33's kn airflow and exhaust , have a programmer , i pull my sleds i get 10, i tell ya what i struggle to get 300 miles to a tank and thats with the low level light comming on. ive just accepted that it is what it is and realize i will never get great mpg out of it....some say they get 18 i just dont see getting 400+ miles outta tank of gas but who knows, id say try some fuel line cleaner, ive noticed it gets a hair more mpg on motorcraft semi-sync oil for whatever reason, a fuel filter and pvc valve are a cheap shot to fix/enhance it as well

 

Clean the MAF sensor with MAF cleaner, you can get it at any wal-mart or parts store.

 

i have a 2001 4 x 4 5.4l and it gets an honest 15 mpg in the winter at 55 mph on stat highway. I fi pull a 4000 trailer it gets about 11. In the summer, when warm, and summer gas blend, it is more like 17-18 mpg. I just have standard intake and program. Run 5 w30 dyno oil. You must have a sensor (MAF) problem or something like that. I have 185k miles and put plugs and fuel filter in about 130k miles.

 

Why a 3" exhaust system? That is part of it I am sure. Good luck!

 

Also, have you changed from stock tire size? If you changed that quite a bit in size, you have 2 issues. First off your speedo is off. So your MPG figures will not calculate right because the distance isn't right.

Second, with bigger tires and 3.55's, your gearing is off from optimal for mileage.

 

My 98 F150 5.4L has a K&N Air Filter (Stock In-take) and a Superchips Micro-Tuner. I don't know my milage because I have 33" Tall tires and my stuff is off like the post under me says.

You can try: changing your air filter to a K&N (You will notice it, I did)
your fuel filter
spark-plugs
Cleaning the Throttle Body with either (make sure its wire/electrical safe)
If you have access to: plug a diagnostic tool into the dash and see if any codes are there (Some codes may be there even without the engine light being one) Advance Auto and Auto Zone will do it for free here.


Trying some of this stuff out may help you out.

 

Anyone think it may be the exhaust size is messing with the oxygen sessors going to the computer? You could test them out and see what happens too. Should be 4 of them.

 

I'm getting 15-16 mpg on my 5.4. Hopefully some of the suggestions will increase your mpg's. I do notice that my mpg's fall considerably if I have the hammer down at 75+. Sounds like you have bigger issues than just how fast you might be travelling.

 

i may have made a typo on the exhaust, i believe it is a JBA stainless steel side exit system... it is likely 2.5inch but for some reason 3 inch popped in my head... how do i check the exhaust sensors to see if they are working properly? i am going to clean the MAF and replace the PCV as soon as i get a little time,

 

Mine is heavily modded and I am seeing 12ish range in the city lately....during the summer I get up to 14-15+ mark. Winter gas is probably the culprit (and longer warm-ups) for low mpg's during the winter.

Efans will get you a mpg or 2 btw.....

 

I have the same set up except a programmer and 2.5" back free flowing system with 35's and I do see the 18 on highway. 17+ for sure. I would guess it is time to do a good thorough tuneup and overall look thru of the truck. clean everything that needs to be cleaned and change all your filters. there is no reason to be getting that low of milage in that truck. you should just about average 14 no matter what.