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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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TXiceman

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Posted: 01/24/21 09:17am Link  |  Quote  |  Print  |  Notify Moderator

Personally, I'd want a 3.73 axle for towing. The torque amplification is more easily handles in the axle than the transmission. Sure you have multiple gears and you can drop down to 4th or even 3rd gear to pull the grade.

Ken


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2013 HitchHiker 38RLRSB Champagne, toted with a 2012, F350, 6.7L PSD, Crewcab, dually. 3.73 axle, Full Time RVer.
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Lantley

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Posted: 01/24/21 11:11am Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

^^^^At 25000 lbs and on 7% grade a 475 hp truck will pull down to about 58 mph. The difference in 3.31 gears vs 3.55 gears is this:
If the truck with 3.55 gears and 34” tires will be running at 2600 rpm in 6th gear it can put out 475 hp.
The truck with 3.31 gears and 34” tires will be running at 2800 rpm in 5th gear at about 56.4 mph and it will be putting out about 462 hp.

But if the hill gets a little steeper the 3.31 ratio equipped truck gains hp as it slows down where as the 3.55 equipped truck will loose power as it slows. So really both gear ratios will perform extremely well with loads under about 18000 lbs. once you get into dually territory 3.31s are no longer as desirable nor are they available.

Are you basing your logic on towing a 25K loaded RV? Not many if any are towing a 25K RV. But maybe you are referring to a 25K GCWR which is a realistic figure.
But if 34" tires are required to prove your point, you are stil not providing real life scenarios.
So far I'll stick by my comments that the 3.55 will deliver a better towing experience simply because it will hold gears a little more while going up the grade and be less likely to downshift. 3.55 may also get you started a little easier.
But if your towing with 34" tires for some reason:[emoticon] 3.31 maybe better[emoticon]


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4x4ord

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Posted: 01/24/21 11:36am Link  |  Quote  |  Print  |  Notify Moderator

TXiceman wrote:

Personally, I'd want a 3.73 axle for towing. The torque amplification is more easily handles in the axle than the transmission. Sure you have multiple gears and you can drop down to 4th or even 3rd gear to pull the grade.

Ken


When your cruising down the highway at 62 mph with a typical rv the torque output of the engine might be about 500 lbft in 10th gear with 3.55 gears vs 536 lbft with 3.31 gears. When you hit a hill and the rear wheel torque demand increases the 3.31 truck will drop to 9th, 8th or 7th before the 3.55. This is desirable. There is no point in putting high power (1000 lbft of torque) through the overdrive gears to speed the driveshaft up just to run that power through a high ratio rear end to slow it down again.


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4x4ord

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Posted: 01/24/21 11:41am Link  |  Quote  |  Print  |  Notify Moderator

Lantley wrote:

4x4ord wrote:

^^^^At 25000 lbs and on 7% grade a 475 hp truck will pull down to about 58 mph. The difference in 3.31 gears vs 3.55 gears is this:
If the truck with 3.55 gears and 34” tires will be running at 2600 rpm in 6th gear it can put out 475 hp.
The truck with 3.31 gears and 34” tires will be running at 2800 rpm in 5th gear at about 56.4 mph and it will be putting out about 462 hp.

But if the hill gets a little steeper the 3.31 ratio equipped truck gains hp as it slows down where as the 3.55 equipped truck will loose power as it slows. So really both gear ratios will perform extremely well with loads under about 18000 lbs. once you get into dually territory 3.31s are no longer as desirable nor are they available.

Are you basing your logic on towing a 25K loaded RV? Not many if any are towing a 25K RV. But maybe you are referring to a 25K GCWR which is a realistic figure.
But if 34" tires are required to prove your point, you are stil not providing real life scenarios.
So far I'll stick by my comments that the 3.55 will deliver a better towing experience simply because it will hold gears a little more while going up the grade and be less likely to downshift. 3.55 may also get you started a little easier.
But if your towing with 34" tires for some reason:[emoticon] 3.31 maybe better[emoticon]


Yes my math is based on 34" tires and 25k lbs combined weight. With a ten speed if you find yourself towing in a situation where the transmission is constantly shifting in and out of 10th you can simply lock out tenth gear ... or even nine and tenth if needed.

StirCrazy

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Posted: 01/24/21 06:46pm Link  |  Quote  |  Print  |  Notify Moderator

lenr wrote:

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.



realy? my manual says to use cruise control as that the only way the enging braking decending hills works. I think you need to read the manual again.

Steve


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Posted: 01/24/21 06:59pm Link  |  Quote  |  Print  |  Notify Moderator

I'm pulling my fiver with 3.55's and it tows like a dream.. Yup, and stops great too ! [emoticon]


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4x4ord

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Posted: 01/24/21 08:34pm Link  |  Quote  |  Print  |  Notify Moderator

StirCrazy wrote:

lenr wrote:

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.



realy? my manual says to use cruise control as that the only way the enging braking decending hills works. I think you need to read the manual again.

Steve


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.

lenr

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Posted: 01/24/21 10:00pm Link  |  Quote  |  Print  |  Notify Moderator

I drive a 2012 Job 2 6.7 diesel. It engages engine braking when tow haul is turned on and no other time. In tow haul if you press the brake petal more than slightly, it will down shift to cause more engine braking. I understand that later years (maybe around 2015) the engine braking and tow haul modes were separate switches so one could engage the higher shift points of tow haul without engine braking involving aggressive down shifts. When tow haul and and cruise are both on the engine will downshift aggressively to try to maintain the cruise speed setting when gaining speed down hill. As said above, I'm in that model year range where engine braking was not very effective. However, if one sets the cruise about 10 mph less than the maximum speed that you want to achieve going down the mountain it will usually hold it back to that +10 setting. At least it will going down the South side of Mount Eagle Mountain. I don't believe that a 2012 does any automatic trailer braking. I have no idea how later years behave, but I cannot imagine that Ford would keep engine braking from working if one was driving without using cruise. Oh, and I was only indicating that I shut off cruise going down short hills where I'm only going to gain a few mph to keep the down shifting to half of what it would be otherwise. It will still down shift going up the other side.

Apologies for getting away from OP's original question. Again, either axle ratio will be fine but I would choose the 3.55 because the 10 speed top gear ratio is so low.

StirCrazy

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Posted: 01/25/21 08:16am Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

StirCrazy wrote:

lenr wrote:

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.



realy? my manual says to use cruise control as that the only way the enging braking decending hills works. I think you need to read the manual again.

Steve


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.


yup, I know how it all works, there actualy was no exhaust brake in the 11 to 14 models it just used the VVT which is why it was ok at best, but having said that it does hold my 5th wheel on a grade 6 hill no issues so it isn't that bad, on a grade 8 I need to tap my brakes every now and then. but the cruse control has to be on for it to work. if I dont have the cruse control on there is absolutly no engine or transmition braking , I dont know if this changed with the 2015 and up models or not. adaptive cruse controle wasn't an option in 2014 on the f350

Steve

StirCrazy

Kamloops, BC, Canada

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

lenr wrote:

I drive a 2012 Job 2 6.7 diesel. It engages engine braking when tow haul is turned on and no other time. In tow haul if you press the brake petal more than slightly, it will down shift to cause more engine braking. I understand that later years (maybe around 2015) the engine braking and tow haul modes were separate switches so one could engage the higher shift points of tow haul without engine braking involving aggressive down shifts. When tow haul and and cruise are both on the engine will downshift aggressively to try to maintain the cruise speed setting when gaining speed down hill. As said above, I'm in that model year range where engine braking was not very effective. However, if one sets the cruise about 10 mph less than the maximum speed that you want to achieve going down the mountain it will usually hold it back to that +10 setting. At least it will going down the South side of Mount Eagle Mountain. I don't believe that a 2012 does any automatic trailer braking. I have no idea how later years behave, but I cannot imagine that Ford would keep engine braking from working if one was driving without using cruise. Oh, and I was only indicating that I shut off cruise going down short hills where I'm only going to gain a few mph to keep the down shifting to half of what it would be otherwise. It will still down shift going up the other side.

Apologies for getting away from OP's original question. Again, either axle ratio will be fine but I would choose the 3.55 because the 10 speed top gear ratio is so low.


my 2014 will engine break/downshift (not sure if it uses the wheel breaks or not but it could. I suspect it doesn't as when I am on the big hills and i do have to brake it doesnt take much to bring me back down. maybe thats only an option with the newer models that have adaptive braking?) when the cruise control is on in tow/haul or normal. towhaul only changes shift points. if I turn off cruise or tap the brakes because I have to wait to pass someone when I am going down a hill it will speed up like crazy. going to the coast in the bigger hills towing my 5th wheel the system can hold me to 60mph but if I tap the brakes or turn the cruise controle off I will be up at 80 or 90 in no time.

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