Dave i have a 2001 expedition with the 5.4 and 3.55 axle. I was getting 13 mpg.(with 500 miles) before i put a three inch exhaust from the y-pipe back and put in a K&N oem filter. I did these mods and my next tank of gas i was averaging 15.5 mpg and that was pretty constant till we got about 1500 miles. We now have about 2100 miles and we just made aroad trip to Dallas to see my inlaws and we got 408 miles on one tank of gas before the low fuel light came on. Filled it up an we averaged 18.6 running 74mph interstate and the last 60 miles on the tank my father-in-law and i wee seeing what kind of power it had, a little hot rodding.
In my opinion yes you will get better milege after the engine is broke in and all the rings are seated. And yes you are to vary engine rpm over the first 1'000 miles and i did exactly as the MANUAL said. Never hot rodded it till after the 1,000 miles.
We are averaging about 15 to 16 city if we drive easy and 19 or better on the hwy. rom the posts i have read i feel fortunate tobe getting that kind of mileage.
The best thing you can do for your mileage is to NOT gun it all over town and try to maintain constant speeds as much as possible. And when you get ready to have some fun just know that your mileage will suffer. Good luck!

 

Hey thanks 01x for the advice. Sounds like you are getting good gas milelage with your 5.4L. I absoultey love my truck. Never could be happier. Also I would like to add I think the Expeditions are the sharpiest suv out there and if I didn't get a truck I would of got Exp, very cool looking just like F-150

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2001 F-150 5.4L trition,
Regular cab with 8' bed
XLT with Black Clearcoat
4x4 3.55 limited slip
factory cab steps
class III tow package
Factory ordered: 12/5
recieved: 2/22

 

I've been getting around 13-15 mph my 1st winter with my 5.4. About 70% highway and 10% 4wd. I'm going to add a tonnaeu cover and see if the mph improves this summer. I really like the low end torque of this engine while towing!!

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00' F-150 XLT 4X4, SC, SB, AT, 5.4, 3.55, Tow pkg, DuraLiner,
Westin platnium bars, , Chestnut Brown/Beige