i have a 5.8L in my truck but other than that it's a lot different, but i still only get 11-12MPG and that is on nice days when there is no show on the road, in bad conditions i get about 8-9MPG if im lucky haha... its a '96 F-150 5.8L E4OD A/T 4x4 w/3.08 open diffs and 265/75R15's which are only around 29 or 30ins. ....
My BIL had a 91 reg cab F 250 4x2 with the 351 and a utility body. It barely got 11 mpg running empty. It had 4.11 gears and 16.5" rims with 80 series highway tires. It did have the HD trailer option for hauling gensets and utility trailers and the gas mileage never dropped much below 8 when towing. IIRC it was rated to tow around 8500 lbs. Very good truck, but the electrical contractor eventually replaced it with a Ford diesel van.
...and just to continue my thoughts.
This has been a BIG pain in my side for the past 3 weeks I have owned this truck - it is costing me a fortune...Wishing I had researched it before I went to the 351.
My efforts are going to be slap a 4r70w in, decent converter, Optishift computer, mustang EEC with MASS AIR. SOUnds like a lot of work just for MPG...but I have other motives as well with that setup.
I will also be ditching the smog stuff, smaller crank pully, electric fan - this will make less rotating mass free up power.
I have hiway gears as well, so I think, have not checked, it cruises at 80 around 1900 rpms...I do mostly city drives, so deeper gears should help really...and with the bigger tires I plan on putting on, will be necessary.
1900rpm at 80mph! Are you serious? if so the gearing is less than ideal or you have oversize tires on it and the speedo hasn't be recalibrated for the tires.
For best milage these motors need to turn about 2000rpm at 70mph give or take, but heavier trucks need more gearing while lighter trucks can get away with less. My 5k extended cab got 13/17 no matter how I drove it with 3.55 gears and 31" tires.. just so you know what is possible.
96 F150, Std. Cab, short box, 4x4, 3.55 LS, stock tires, unloaded I (used to) get 15-17 highway on my commute during the summer, down to the neighboorhood of 12 during the winter. Hauling my toolbox and belongings in a trailer from Michigan to Washington state this December, 9-11 mpg. Across the plains, with a headwind, up mountains, overdrive on or off, didn't matter, 9-11 all day. Now that I'm out here, and I drive 2 miles to work in the city so the poor thing never gets warmed up and I am pretty sure a head gasket started weeping somewhere in Iowa, it's so bad I don't even calculate it. This summer, driving to Kentucky, all highway, unloaded, I got almost 20 (19.5) Honest! Than again, that is using the completely innacurate "Divide mileage from last fill up by gallons it took to fill back up" method. Not an exact science, but close enough for gov't work...
My 92' F-250 351 ZF5 4x4 with 355's gets a consistent 12/16. I actually think the higher gears are hurting my mpg's. If I'm starting off on an incline at all, I have to use 1st. gear. Guess it's a good thing that this is my Sunday Go Ridin' truck...lol.