I have been dying to ask this question.
How does this engine pull grades under a load with the 3.31 Diff, as compared to the 3.73?

I'm looking to maximize fuel economy.

What kind of RPM's are you running at highway speeds.

Thanks.

 

It somewhat depends on what you're towing and how fast, but most reports are that it's just as good as the 3.55s due to the 6-speed transmission being nearly always able to put the engine exactly where it needs to be.

I think this is the reason the towing capacity is identical between both ratios. I had the 3.55s and wished I had the 3.31s!

 

I think 3:31's are a great gear, if the truck needs to downshift it puts me at a really nice RPM for maintaining 70-75 mph, but keep in mind I have the stock 245's on 17" wheels, I would say if you wanna run over size tires get the 3:55's otherwise check back with me in a month or so after I add 275/70 18's

 

I have the 3.31 with 18 inch wheels and I don't see how I could overload my 3.31 gearing staying within the legal limits of an F-250.
I'm certain it would fail me if I tried to move 25,000 lbs up a mountain but that's because my F-250 isn't rated for that type of weight.
55 MPH empty in 6th is about 1,200 RPM to give you an idea.

If you plan to go F-350 SRW, get the 3.55.
The MPG difference isn't much from the numbers I see posted here.

 

I have the 3.31 gears at 75 on a 7% grade pulling 8000lbs never droped to 5th.I just have the stock tires so not sure how it would do with bigger tires. My truck is 4 door short bed 4x4. XLT

 

I have the 3.31 gears and it depends on what speed you like to pull at. I found running 63 to 65 pulling about a 10,000 lb 5er that it works well. Doesn't drop to 5th except for big hills with lots of wind and the best fuel mileage. If I ran at 55 to 60 then it was dropping into 5th more often and to me it seemed to lug more.

According to other posters here 70+ works good too.

The 3.31's are really nice empty, I find I can run 70+ and get some good fuel economy.

 

I have 331's with 18 inch wheels and I pull a 11,000 pound 5th wheel with ease. I run at around 62 MPH with the same results as Whiteline. At around 70-74 MPH empty I get 18 miles per gallon. I am very happy with the gear and tire combination.

 

Thanks everyone who responded.
I'll be pulling a 5er, probably 12-15000 loaded. I don't expect to race up hills without loosing speed or dropping down gears.
I would only be pulling the trailer about 10-20% of the time, so the rest of my miles I want to maximize my fuel economy. So it's a trade off.

I just wanted some real world info on how these trucks really pull.

I've run every steep grade in america in tractor trailers, 10, 13 and 18 speed, but never an automatic. So thanks for your help.

I have to confess that I don't really understand the tire size issue...17 vs. 18 inches though. Duhhh!!!

 

17 vs 18 inch wheels that is.

 

The tire diameter is almost the same 17 vs 18. and with 20's you will get 3.55 gear so end results are about same. gear ratio and tire rollout. now this is with stock oem tires.

 

With the mini van wheels....

 

I have 3.31 gears in my F-350 and see no reason to buy 3.55 gears with this truck.

 

Then you should be able to appreciate that the rear end gears only tell part of the story. My last two KWs had 3.25 rears!

It's funny you mention that, Gavin! As much as I made fun of my 17" wheels I never really thought much about it. And then it hit me: My wife's Sienna has 18" wheels, and my F350 had 17s....Not much for that "macho" look, but it makes for some nice and cheap tires. Unlike the van and it's damn run-flats...

 

I have 3.55s and seem to be getting similar mpg as the 3.31 numbers.