After looking at the stickers on fifty or so window stickers. There is a 3.55 and 3.31 rear ends, for someone that will be pulling 16,500 lbs, once in a while. Which would be best. And still get the best fuel mileage while not towing.

Also electronic locking is referring to the front axle? And I guess if they don't have that, you have to get out of the truck and turn the hubs on the front wheels to put it in four wheel drive? Thanks guys!

 

Personally, I would go with the 3.55'S but there may be others that chime in here.
Electronic locking IMO is referring to the electronic locking rear differential. I had it in my 2011 F250. I was sweet when one wheel didnt have traction and one wheel did. I didnt use it a lot, but was there when needed!
The front hubs are vacuum operated when switching from 2 wd to 4 wd.

 

I have 3:31s.... My unloaded mpgs are usually any where from 18 to 21 mpg depending on conditions and speed. I don't have any trouble towing the rv either.

Personally Im not sure it matters much any more due to the different tire size options with the gear sets. The final drive ratios are pretty much the same.

 

I opted for the 3.31 rear end, where I'm at is flat as flat as can be with the only elevation changes are overpasses. These trucks have so much more power than their predecessors I decided that if I am ever towing heavy in hilly terrain just drop down into 5th gear. I recently went up north with my snowmobile trailer in tow, 70 mph = approx 1600 rpm, I have the 18" wheels.

What was noted above is correct the electric locking diff refers to the rear end vs the older style mechanical.

 

The 3.55 is as low as you can go and get with the electronic locking rear differential. I get good gas milage with the 3.55 and like having the reassurance of a locking diff if you ever need it. The front diff is open, only way to get a locking front diff is aftermarket.

 

I have the 3.31 electronic locking rear diff if my '15 F350 SRW.

I have a friend who worked briefly at a competing local dealer, as a favor I stopped in to get a price quote and they told me it wasn't available in the configuration I wanted also, but my dealer ordered it without issue.

 

I like the 20" wheels and I believe 3.55 is the lowest available for the 20" wheels.

 

Yeah not so true buddy. 3.55s are the lowest you can get with 20s. You can get 3.31s with the electronic locking axle all day from 2011 to 2015.

 

right, its

3.31 and 18 inch wheels
3.55 and 20 inch wheels
3.73 and 17 inch wheels for the Dually

on SRW trucks, both have the option of a Locking rear diff

 

OOH, thats the way it works. I guess you could order anything you what though. I didn't notice the correlation on the window stickers. I'll do some more digging. The way I see it is if you bigger wheels the tires are shorter. And vise verse, but I always thought that the out side diameter of tire and wheel were the same.

So the 3.55 are the lowest you go on SRW?

 

The outside diameter isn't the same, its close but not the same as if you want to change wheel size you still have to recalibrate speed/odo.

As I stated above I have a cclb with 18" wheels, SRW, 3.31 electric locking rear diff. It really doesn't take that long to order what you want vs. finding one close to what you want in stock somewhere.

The last three trucks I have bought I ordered to my specs, that way I get exactly what I want.

 

I'm not sure if you can get 3.73 in an SRW diesel truck? I think the gasser trucks can go with a 4.10 even? I'm not sure but it should be easy to see all that stuff on the ford website.

It depends on what wheels you like. Sadly, the styling of the 18" and 20" wheels will likely decide which rear gear you want. Can't get the 3.31 with 20" wheels. You can get 3.31 or 3.55 with the 18" wheels. I think you can get 3.55 or 3.73 with the 20" wheels.

For what you describe, you don't need a short gear, so, just go with which wheels you like. If you tow 16,500 every week all year long I'd say get 3.73 or 4.10, but that would be for a work truck, not a personal vehicle.

 

With a 16500 lb trailer your GCW will most likely exceed 23500. A drw with a 3.73 will get you over 31k on GCW. Loaded truck hooked up will be +-13K plus trailer will be +- 13K for a GCW of 26K.

 

My experience:

2011 F-350 SRW CC LB 3.55 Elocking
Tow a 42 ft Raptor. 17,500 lb loaded for camping. (18K GVW 5th wheel)
Truck........8320 lbs GVW ready to tow...full fuel & passengers.

Towed about 4000 miles. No problems at all. Over the rockies in Montana and the cascade mtns of Oregon. Plenty of power. (could have used a good exhaust brake)

Problem:
1....Pin weight of 4380 lbs put too much weight on the truck.
2....GCVW....Truck and 5th Wheel = 25,560 lbs

Those are 2 specifications that I exceeded. The truck did very well. BUT.....Long story, short.......I have a New 2015 F-350 DRW CC LB 3.73 limited slip. (link in my Signature)

I have not towed yet. Light.....the truck drives so so very nice and does better in the ice and snow that the old one.

 

This only applies when your truck has the Electric Shift Transfer Case. The manual shift has manual hubs.
Just to be clear for the OP.