Ford super duty 3.31 rear end. Will it be a big mistake. ?

What I don't understand why buy a truck and have it downshift with 3.31 gears when it would hold the gear its in with 3.55 gears? What's the logic in buying a truck for TOWING that will do more down shifting vs. one that would perform better while towing?
What am I missing?

I run 275/R65-18 highway tires, so the 3.31 works (mostly) for me. My 800 torque and 400 hp works great as others have said above. Tow haul mode absolutely eliminates gear hunting—my transmission just drops to the gear needed and holds it. My comment earlier about dropping to fifth on hills was based on experience in states with rolling hills where little grading is done. My favorite complaint is I-26 in NW SC. The trans will drop to fifth going up, back to 6th as I crest, and shift back down to fifth going down with cruise control trying to hold the speed. Just a ton of shifting! Yes, I know the owner’s manual says to not use cruise when towing. I end up shutting cruise off going downhill letting it coast up a few mph.

Just to answer the OP’s question clearly: no you’ll not make a mistake with 3.31.

StirCrazy wrote:

lenr wrote:
My bad on 20" tires requiring 3.55. Looks like the last year for that was 2019. I still wouldn't choose it for pulling a DRV. Not that it matters here, but the 6R140 does not have a 1:1 ratio. 6th is 1:1.15. I believe the Allison 1000 does have a 1:1 ratio.

Im going to have to go look at the build sheet in my 2014, I have the 11500 gvw with the 20" wheels but I was sure it said I had the 3.31's
. I have to definatly check that. all in all dropping a gear out of OD on a steep climb is what you should be doing anyways for better efficiency , I do all the time on big hills, on the 10% grades I may drop two, but I am pulling a little more weight.

Steve

There's a couple grades I pull every year ,I just drop it in to 4th ,and let her run . I don't push either one , run them about 55 mph.

lenr wrote:
My bad on 20" tires requiring 3.55. Looks like the last year for that was 2019. I still wouldn't choose it for pulling a DRV. Not that it matters here, but the 6R140 does not have a 1:1 ratio. 6th is 1:1.15. I believe the Allison 1000 does have a 1:1 ratio.

Im going to have to go look at the build sheet in my 2014, I have the 11500 gvw with the 20" wheels but I was sure it said I had the 3.31's
. I have to definatly check that. all in all dropping a gear out of OD on a steep climb is what you should be doing anyways for better efficiency , I do all the time on big hills, on the 10% grades I may drop two, but I am pulling a little more weight.

Steve

My bad on 20" tires requiring 3.55. Looks like the last year for that was 2019. I still wouldn't choose it for pulling a DRV. Not that it matters here, but the 6R140 does not have a 1:1 ratio. 4th is 1:1.15. I believe the Allison 1000 does have a 1:1 ratio.

^^^^ With the 10 speed 3.31 gears are available with 20’ wheels in the SRW Ford trucks. Dropping gears on hills is no big deal. In fact it makes sense to be in direct (4th gear with the 6 speed or 7th with the ten speed) when you’re pulling grades. The additional torque of the newer engines coupled with the lower 1st gear ratio of the 10 speed transmissions make 3.31 gears an excellent choice for lighter trailers. ( by lighter I mean under 20,000 lbs). Once you go dually with the Ford, 3.55 becomes the highest ratio offered.

It would be handy for folks to read the 2021 Ford Super Duty Order Guide when determining axle ratios. The built my truck app on line will show ones choices as you build. Ford is fairly stingy on axle choices depending on model and options. For Instance if one gets 20” SRW tires, only the 3.55 may be selected with a diesel. We tow with a 2012 F-350 diesel 3.31 6R140. When we had a 9400 lb fifth wheel it was just fine. Now that we have upped to 12,700 lb, I wish I had a 3.55 because I drop to fifth going up hills in those states that don’t grade much. As said above, desired axle ratio is dependent on tire size and top transmission ratio. The top ratio of the 10R140 is about 6% lower than the top gear on the 6R140 and the 3.55 is about 7% higher than the 3.31 so I would definitely get the 3.55 given the choice—same engine RPMs at highway speed. That said, if buying off the lot and other things dominate the choice, the 3.31 will just run in lower gears. The transmission is a truck transmission and is built to run in lower gears all day if needed. 9th gear is just 9% lower than 10th.

I have the 3.31s in my f350 and pull a 14K 5th wheel. no problems at all, I guess if you need to do stop light racing with the 5th behind it you could go 3.55s haha.. seriously though most of my driving while towing is on the highway so it works great for me. what little city areas I do have to go through I just drive. still pulls harder than most cars...

Steve

If it turns out that bad the gears can be changed.

4x4ord wrote:
I've got a 2017 F350 with 3.55 gears and 20" wheels that I tow a 16k lb rv with. My 2021 will be 3.31 gears. The 10 speed has a pretty low ratio reverse which is going to be a downer even with 3.31 gears.

Thats good to know my son is about to order a 2021 . He wants the short bed 350 . He likes that Lariat ,with the Tremor package ,whatever that is :) Good friend of mine went with the Lariat with the Tremor package ,nice truck. My son plans on towing a fifth wheel , I believe its in the 14-14,500 GVWR range. I suspected those 3:31's would be a good choice with that 10 speed . I would think if anyone knows you do, so its going to be 3:31 for you for sure ??

Sounds like its a good choice for the OP also . I know I can't compare my truck with 3.42 gears, but even with a 6 speed , and as said 15,500 GVWR , probably right around 13,500 -14K loaded I wouldn't ,and don't need any lower gears . That Cummins does just fine .