I've got a 97 F-150 4x4 EC 4.6 Auto with 3.55LS.

In town I get around 500km to a tank and about 750km to a tank on the highway. These numbers drop about 20%-30% during really cold weather.

That works out to about 305 miles for city and 460 miles for highway. Highway speeds are usually around 60. City driving is not with a real heavy foot. Periodic WOT runs do happen on occasion

I guess with a 25 gallon tank that's about 12 mpg in city and about 18 on the highway. Actual numbers are probably a little higher as I usually fillup with 1/8th of a tank left . . .

 

I think you need to compare apples to apples here. You just can't say 5.4l vs. 4.6l...you have to specify equipment and whether 2D or 4WD, rear axel, etc.

My 97 4 X 4, 4.6l with 3.55 axel only gets 14-15 MPG all around, which I think sucks. I did get 17 MPG on a long trip once, but that's about the best.

You also have to consider the efficiency of the engine with the vehicle it's pulling. I once had a '66 Dodge Charger with a 383 4 spd that got better mileage than a friend's Charger with a 318-2 barrel. I think that the 318 was just too small to haul the vehicle around very efficiently. So, for my $.02, I'd pick the 5.4l if I had a second chance as the mileage is comparable, but it's somewhat snappier, performance-wise.

 

Taking the 4.6L engine and fitting a stroker crank to end up with 5.4L increases maximum cylinder volume, which in turn increases the amount of air/fuel mixture drawn in on the 2nd downstroke.
This is simple math, and it means that the larger 5.4L engine will always burn more fuel than the 4.6, given similar states of tune and environment (altitude, temperature etc).
The 5.4L pays off with more maximum torque available (and therefore Hp) in heavy towing applications...a benefit of any stroked engine (including my Harley).
A 5.4L F150 might approach the fuel efficiency of a 4.6L if you switched the final gearing to something like a 2.90, which would allow the increased torque of the larger engine to be of benefit in everyday driving...but this might require a revised camshaft to bring the powerband down...and this could cause a loss in efficiency!!...it's a no-win situation.
With the exception of the niche RV towing community, truck engines are getting smaller, and doing more with less.

 

jaymz is right - too many variables to compare when eveyone throws their numbers out. A comparison between two simalarly equipped F-150s is the only way to truly get a good comparison. Heck jaymz and I drive "similar" trucks, except his is a 4wd vs a 2wd, 3.55's vs 3:08's, and different tire sizes. His mileage is much lower than mine. My long term average over 5 plus years is right at 17 mpg. On the freeway doing 75, I can get well over 20 mpg (been up to 22 mpg).

We have the truck that replaced my truck still in the family. The ONLY differences are mine is 2wd, that one is 4wd, mine is a '97, that one is a '99, I have 3:08's, that one has 3:55's, and mine is Moonlight Blue, that one is deep Wedgewood Blue. That truck averages 15 mpg over three years, I'm at 17 mpg. We both drive about the same and rack up the same amount of miles in similar driving. (50/50 city/freeway)

My personal feel is that fuel mileage on the 4x4's is closer to being a wash between the 4.6 and 5.4 due to the extra weight and drag of the 4wd drive system. Same reasons jaymz pointed out in the old dodges- the smaller engine just has to work harder. With a 2wd, I think the 4.6 does better in the fuel mileage department, though it gives up power to the 5.4 (duh!). I'm sure if you do a search, you can find someone who has a truck the same as what you want and a similar driving style. Good luck!