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

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Lantley

Ellicott City, Maryland

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Posted: 01/23/21 01:12pm Link  |  Quote  |  Print  |  Notify Moderator

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?


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fowlthing

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Posted: 01/23/21 02:29pm Link  |  Quote  |  Print  |  Notify Moderator

My thought is that while in tow haul mode..it won't downshift excessively ... and provide better fuel mileage... most of my miles are not loaded...

But you make a good point if you buy it to tow...then get something to tow with... if a person wanted to save money.. don't buy a camper to begin with..lol.


So far most of the people have been happy with the 3.31


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Lantley

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Posted: 01/23/21 03:05pm Link  |  Quote  |  Print  |  Notify Moderator

fowlthing wrote:

My thought is that while in tow haul mode..it won't downshift excessively ... and provide better fuel mileage... most of my miles are not loaded...

But you make a good point if you buy it to tow...then get something to tow with... if a person wanted to save money.. don't buy a camper to begin with..lol.


So far most of the people have been happy with the 3.31


Ok the idea is better MPG's. Maybe if I never towed 3.31 would be preferred. However given a choice for someone who TOWS 3.55 will always provide more towing performance.
MPG difference between 3.31 and 3.55 can't be much?[emoticon]
I guess you could make the same argument that towing performance improvement between 3.55 vs. 3.31 can't be much!....[emoticon]
Either way I'll go with towing performance. A diesel pick up will never deliver great MPG's. However I want all the performance I can get when pulling a long steep grade.

justme

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Posted: 01/23/21 03:14pm Link  |  Quote  |  Print  |  Notify Moderator

It is all about torque. The torque multiplier is accomplished by the differential and transmission. The higher gear ratio of the differential buts more demand on the transmission to produce the torque for high loads. That means the transmission works harder to make up the torque. So in a practical sense, it boils down to how often the truck is required to pull heavy loads vs no loads. A good indication is how often the truck shifts gears to maintain the speed limit with normal driving. Diesel trucks are not as sensitive to torque demands as a gas truck.

4x4ord

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Posted: 01/23/21 06:36pm Link  |  Quote  |  Print  |  Notify Moderator

Lantley wrote:

fowlthing wrote:

My thought is that while in tow haul mode..it won't downshift excessively ... and provide better fuel mileage... most of my miles are not loaded...

But you make a good point if you buy it to tow...then get something to tow with... if a person wanted to save money.. don't buy a camper to begin with..lol.


So far most of the people have been happy with the 3.31


Ok the idea is better MPG's. Maybe if I never towed 3.31 would be preferred. However given a choice for someone who TOWS 3.55 will always provide more towing performance.
MPG difference between 3.31 and 3.55 can't be much?[emoticon]
I guess you could make the same argument that towing performance improvement between 3.55 vs. 3.31 can't be much!....[emoticon]
Either way I'll go with towing performance. A diesel pick up will never deliver great MPG's. However I want all the performance I can get when pulling a long steep grade.


How will 3.55 gears perform better than 3.31 gears on a 7% grade towing a 17k fiver? (A 7% grade should be steep enough to slow the Powerstroke down to about 58 mph towing at 25000 lbs combined weight.)

* This post was edited 01/23/21 06:52pm by 4x4ord *


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Lantley

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Posted: 01/23/21 06:44pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

Lantley wrote:

fowlthing wrote:

My thought is that while in tow haul mode..it won't downshift excessively ... and provide better fuel mileage... most of my miles are not loaded...

But you make a good point if you buy it to tow...then get something to tow with... if a person wanted to save money.. don't buy a camper to begin with..lol.


So far most of the people have been happy with the 3.31


Ok the idea is better MPG's. Maybe if I never towed 3.31 would be preferred. However given a choice for someone who TOWS 3.55 will always provide more towing performance.
MPG difference between 3.31 and 3.55 can't be much?[emoticon]
I guess you could make the same argument that towing performance improvement between 3.55 vs. 3.31 can't be much!....[emoticon]
Either way I'll go with towing performance. A diesel pick up will never deliver great MPG's. However I want all the performance I can get when pulling a long steep grade.


How will 3.55 gears perform better than 3.31 gears on a 7% grade towing a 17k fiver?

Believe me I'm no expert. But the higher ratio always provide more torque to the wheels. Granted it will also use more fuel.
In the old days before multi speed transmissions one would use 4.10, 4.3 4.56 gears to improve towing performance. Now with multi 10 speed transmissions. The ratio is less critical become the tranny has more gear to adjust to nevertheless it is still my understanding that higher ratio will provide more power at the wheels all other factors being equal.

4x4ord

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Posted: 01/23/21 08:46pm Link  |  Quote  |  Print  |  Notify Moderator

^^^^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.

hondapro

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Posted: 01/24/21 05:30am 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.



4X4ord nailed it either ratio will work great.I doubt you would notice much difference between the two when driving.


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Lynnmor

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Posted: 01/24/21 06:20am Link  |  Quote  |  Print  |  Notify Moderator

The 3.55 is better for the transmission, it ain't about the drivers comfort or convenience. I wouldn't want to pay for repairs on a 10 speed transmission.





philh

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Posted: 01/24/21 08:29am Link  |  Quote  |  Print  |  Notify Moderator

Lynnmor wrote:

The 3.55 is better for the transmission, it ain't about the drivers comfort or convenience. I wouldn't want to pay for repairs on a 10 speed transmission.

Why is 3.55 better for the transmission?

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