I have a 2003 FX4 with the 5.4 and the rear end code is H-9. From what I find, that's the 3.55 gear ratio. I tow 2 jet skis quite often to Roosevelt Lake Arizona from Mesa where I live. Total weight of the skis and trailer is around 2500 lbs. There are several grades exceeding 6%. There are times where I'm doing 55 in 2nd gear to make the grade. I could easily go faster but I drive pretty conservatively.
Is this normal or should I expect better?

 

I towed a single jetski (3-seat wave runner) for one summer with my 2003 FX4. Last time I drove that truck was 2011 so my details are shady, but from what I remember the truck had more power than a 2nd gear highway pull with that small of a load.

I can imaging the truck is screaming at 55mph in 2nd?

I guess a good question to ask is tires....what size tire on the truck?

For the record, I had a smaller truck (mid size) the following year. There was one place I'd go that dumped you onto a steep uphill grade of 6% from a stop. The smaller truck struggled to get moving, but it wasn't a 5.4 motor either. I don't recall the Ford having issues here at this place.

 

what is the rear axle ratio? 'im assuming it has 3.55s. which is the lowest you could get besides 3.73s on 7700 models. 6% grade is steep though. is 55 as fast as it'll go wide open on those grades? that however shouldn't be the case

 

I think my truck was the 3.73 gears. I remember that truck was one of the most snappy vehicles I'd ever owned. I think it was a 7700 model. Had the large rear end.

 

I had plenty of power and the rpms were only about 3200 if I remember right. I just thought it odd that it wouldn't hold 3rd gear. I could easily do 65 or better in 2nd gear but it's an old truck and I didn't want to push it too much.

 

The tires, as is the entire truck is bone stock. This one has the 17 inch wheels. Tire size is 275/70R/17.
I'm towing two of the 3 seat jet skis plus the double trailer and it wasn't screaming at all. The truck seemed to handle it ok other than downshifting to 2nd.
I have held it at 75 in 2nd up those hills once and yes, that time it was screaming.
Also, it's been well over 100 degrees here lately so the heat was in the mix as well.

 

100 degree heat might have something to do with it. less oxygen in the air than when its cold. sounds pretty normal to me. i wouldn't stress it. let the truck do its thing

 

Stock tires size for the Fx4 is 265/70/17, but 275 is only ~2% larger, so should not make that much of a difference.

 

I stand corrected. You are right, sir. My tire size is 265/70/17. My mistake.

 

The diffrential should have a tag under a cover bolt to confirm 3.55. A bolt might cover the 3. 265 to 275, 1.7" circumference increase, yes about 2%.