There's a thread floating around here somewhere and guys are getting close to 20mpg out of their modular v8 trucks, but most seem to get in the mid teens. I've got 250k on my truck, and I just replaced the 5.4 that was in it. The original one looked fine externally. No rust, no rot. The used engine I put in it looked much worse due to the fact it came from up north (road salt), but runs like a top. The rest of my drive train is all original, and it is also in excellent condition. No slack in the differentials, no slipping in the tranny, and the transfer case works fine. And don't get me wrong, this truck has NOT been treated well. It's spent as much time off road as on. And it gets 11mpg average. I haven't tested the new motor through a whole tank on the road to see what it's best might be, but I bet it'll do 14 or 15. And I'm running 33x12.50-17's and 3.55 gears.

My 01 got about 14 to 16 most of the time. Supercab F150 4x4 5.4L 4R70W, 3.55 gears with 265/70r17's. Perhaps you should invest in a Diablosport Predator. I had one when I had the 01 and had more power and better mileage.
http://i275.photobucket.com/albums/j...1/DSCN0308.jpg

not a truck i know , but i say you also should invest in a tuner also and see what happens . i have a 95 4.6 bird i bought for 200 and beat the tar out of while my 53 is down . the p.o put a chip / module in it . ford claims high teens / low 20's for mileage . i get roughly on average 23 - 24 in town and 28- 30 on the road . i don't baby it either ! i have found that erreant throttle happy teenager was only hiding , not gone after driving it for awhile .it'll do things my beloved flatty just can't do . }>

You got that right. The 5.4l aftermarket (especially 2V thanks to the gen2 lightning) is huge. Must not know how to use a search engine.:-whud
http://www.johnnylightningperf.com/
http://www.teamjdm.com/

Just sold a Diablosport Predator and the canned tunes were the worst I've come across. SCT Xcal with custom tunes from a good shop is the way to go. Diablopsort has a very good software and tuner but you need custom tunes from a good shop if your going to run one, the included canned tunes are terrible and knock like hell.