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Open Roads Forum  >  Fifth-Wheels

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

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4x4ord

Alberta

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Posted: 01/25/21 08:33am Link  |  Quote  |  Print  |  Notify Moderator

3.31 and 3.55 are both good ratios for towing anything a SRW is capable of towing. Maybe a good way to compare the two is like this:

With the 3.31 ratio the truck will spend a little more time in each gear than a 3.55. (The travel speed range of each gear is about 7% more for each gear with a 3.31 final drive) This means there is apt to be times when it is desirable to lock out 10th gear with the 3.31 gears (maybe towing into a stiff wind). When operated as a 9 speed the 3.31 will do less shifting than the 3.55 equipped truck. When towing light or running empty the 3.31 gears will provide better fuel economy. I think even if I was planning on 90% of my truck’s use to be towing a heavy RV I’d have ordered 3.31 gears and be prepared to lock out 10th gear under certain conditions.


2021 F350 SRW Platinum short box 4x4.
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2008 Citation Platinum XL 34.5

wintersun

Monterey

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Posted: 01/25/21 03:21pm Link  |  Quote  |  Print  |  Notify Moderator

Ford in its trailering guide provides a GCWR that is 6500 pounds greater with 3.55 gears as compared to 3.31 gears. So the gears do matter.

fowlthing

Roger, AR

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Posted: 01/25/21 04:21pm Link  |  Quote  |  Print  |  Notify Moderator

they do for the f250 but for the f350 the GCWR are the same

interesting


2017 Ford 350 SRW
Grand Design Solitude 375RES
may switch to a Ram

JIMNLIN

Oklahoma

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Posted: 01/26/21 06:29am Link  |  Quote  |  Print  |  Notify Moderator

Fords 3.31 vs 3.55 gears with the torqshift 10 speed @ 60 mph on 33" tires shows
7th gear = 164 rpm difference
8th gear = 125 " " ".
9th gear = 99 " "".
10th = 93 " ""

I doubt the 6.7 diesels big HP/torque will notice much difference.

Now the 7.3 gazzer may work a bit better with the 3.55 gears.


"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers

'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
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4x4ord

Alberta

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Posted: 01/29/21 07:57am Link  |  Quote  |  Print  |  Notify Moderator

^^^^ Another way to compare the engine rpm at 60 mph is like this:

3.55 gears with 34” tires 1330 rpm in 10 th gear at 60 mph
3.31 gears with 34” tires 1348 rpm in 9th gear at 60 mph

With 3.31 gears you get an extra gear (10th) to use when towing on flat ground or while running empty.

GIB2

Welland

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Posted: 01/29/21 11:00am Link  |  Quote  |  Print  |  Notify Moderator

GM makes the decision for you . Prior to 2020 the ratio was simply3:73 and 2020 & 2021 with 10 speed 3:42 I pulled every mountain from Buffalo NY to California and Buffalo to Florida through every mountain at 68 MPG. No tuners, no DEF delete , No air bags Buffalo to Florida 8.9MPH and Florida to Buffalo 8.9 MPG .Not towing 20 MPG. I pull a 19,000 pound Mobile Suite fifth wheel. Just fill with fuel and do regular maintenance get in and enjoy.

blofgren

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Posted: 01/29/21 07:05pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:



The Ford exhaust brake started out from 2011 to 2014 as poor. However, with enough engine rpm the 6.7 would hold back a decent size RV. It really doesn't matter whether you've set the cruise or not, what mattered was that the transmission was in a low enough gear to get the engine rpm up high enough to hold back the load. If you find yourself braking too much you need to slow down enough to get the transmission to drop another gear.
2015 and 2016 6.7's have a different turbo that provides much better exhaust braking but the principle is the same .... if the engine can't hold you back in 4th or 3rd you need to slow down enough to get the transmission to drop to 3rd or 2nd.

The Ford exhaust brake has not changed from 2017 until present. It still relies on high engine rpm to provide effective braking. The 2017+ system has a full on and an auto setting. The auto setting attempts to build only enough back pressure to hold the truck at the speed it was travelling at when the accelerator or brake pedal was last released. If the engine rpm reaches redline the wheel and trailer brakes come on periodically to slow the truck down a few mph before releasing again. The computer monitors the amount of automatic wheel braking being performed and determines an approximate temperature of the brakes. If the the brakes get hot it will stop automatic braking and warn the driver of the condition. When the cruise control is used on the Ford trucks with adaptive cruise control the wheel brakes are automatically applied and work in conjunction with automatic downshifting of the transmission and exhaust braking.


If you Ford guys towed with a Cummins you would be absolutely amazed.....I usually have to give my truck some throttle when descending BC passes when pulling my 16k fiver loaded for a 3 week trip. And no cruise control or automatic braking of the service brakes.

* This post was edited 01/30/21 08:55am by an administrator/moderator *


2013 Ram 3500 Megacab DRW Laramie 4x4, 6.7L Cummins, G56, 3.73, Maximum Steel, black lthr, RAM 20k sliding hitch, Retrax, Linex, and a bunch of options incl. cargo camera
2008 Corsair Excella Platinum 34.5 CKTS fifth wheel with winter package & disc brakes

4x4ord

Alberta

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Posted: 01/29/21 11:36pm Link  |  Quote  |  Print  |  Notify Moderator

I think the Cummins holds higher back pressure so it might provide adequate braking without the need to rev the engine beyond 2000 to 3000 rpm. The Powerstroke red lines at 4000 rpm and to provide the level of braking the Cummins does it needs to be revved higher. The exhaust brake on the 2015+ Powerstroke works extremely well with lighter trailers such as our 16000 lb rvs. You simply set it to auto and start descending the grade, the back pressure varies to keep your speed constant. If you want to increase your speed slightly you can touch the throttle and the new set point is whatever your speed is at the moment you let off the throttle. If you want to slow down you can step on the brake and the new set point is whatever the travel speed is when you let off the brake. Automatic wheel braking isn't used until engine redline is reached, which is very seldom with a 16k trailer. I haven't driven the 10 speed yet but having more gear choices will allow us to dial in our speed on the downgrades even better.

* This post was last edited 01/30/21 08:54am by an administrator/moderator *   View edit history

hondapro

Central Pa

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Posted: 01/30/21 04:53am Link  |  Quote  |  Print  |  Notify Moderator

JIMNLIN wrote:

Fords 3.31 vs 3.55 gears with the torqshift 10 speed @ 60 mph on 33" tires shows
7th gear = 164 rpm difference
8th gear = 125 " " ".
9th gear = 99 " "".
10th = 93 " ""

I doubt the 6.7 diesels big HP/torque will notice much difference.

Now the 7.3 gazzer may work a bit better with the 3.55 gears.
.


This is correct, either ratio will work great with the 6.7. You will not notice a 100-200 rpm difference.
Sure on paper the 3.55 will pull slightly better, but seat of the pants while driving they will feel the same.


Steve
2016 F-350 6.7 Diesel
2015 Springdale 311RE

Lantley

Ellicott City, Maryland

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Posted: 01/30/21 08:25am Link  |  Quote  |  Print  |  Notify Moderator

I 'll take slightly better over slightly worse every time.
I'm not sure why some feel the need to go through a lot of mumbo jumbo and tech speak to reach the slightly better conclusion.
The higher ratio will always tow better.
3.31 and 3.55 are pretty close to start with but the higher ratio still performs better when towing. Calculating a performance standard that uses 30" tires is misguided and misleading. Very few are towing RV's with 30+" tires

* This post was edited 01/30/21 08:56am by an administrator/moderator *


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