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Open Roads Forum  >  Tow Vehicles

 > 3.31 gears at 40,000 lbs gross.

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ShinerBock

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Posted: 05/22/21 02:02pm Link  |  Quote  |  Print  |  Notify Moderator

RoyJ wrote:

ShinerBock wrote:

[image]


They say a picture is worth a 1000 words! That's EXACTLY my point earlier:

RoyJ wrote:

That means the ONLY difference in stress between a 3.31 and 4.30 truck is the transmission output torque, or driveshaft torque (again, T-in must = T-out). Because on the same hill, a 4.30 truck may be in 6th and the 3.31 truck in 5th.


Your only stress increase with taller gearing is driveshaft torque.

If anything else is burning up - that's drive abuse. Which granted, happens often with today's fleet drivers. But we're talking RV owners here.


This is referencing drivelines connected to manual transmissions. Eaton and Meritor, class 8 transmission and clutch manufacturers, also did research on the effect of downspeeding on their clutches and manual transmissions, but I cannot find the pdf on that right now.

They had to beef up their products to handle the effects of downspeeding. If I remember correctly, the clutch material had to be changed to handle added stress and a different diaphrag is used to mitigate the vibrations that downspeeding caused. Eaton came out with their Endurant line of transmissions which is more robust to handle the added stress.

In an automatic, you have clutches linking one side that is from the engine(input) and the other linked to the driveline(output) that this stress is being placed upon them. So any added stress is reverberated on the output side into these clutches.

It is partly driver error because you have many people who just let the computer do all the work versus people like the OP you knows that the computer doesn't always know the best gear for what he is pulling and changes gears accordingly to lessen this stress.

Is this driver error? There are arguments for both sides, but I, like the OP, choose to change the gears myself based on what I am pulling. However, there are many out there that don't and just let computer do all the work. These are the people will generally have transmission clutch issues later on.

* This post was edited 05/22/21 02:16pm by ShinerBock *


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larry barnhart

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Posted: 05/22/21 03:42pm Link  |  Quote  |  Print  |  Notify Moderator

When we tow up a mountain our truck wants to keep pulling but I feel better going to 4th gear and let the engine run more RPM's. not the same truck as the posts are telling about but have not had any driveline issues on any towing truck we have had. chevman


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Posted: 05/22/21 03:45pm Link  |  Quote  |  Print  |  Notify Moderator

RoyJ wrote:

blt2ski wrote:

Many of you are also forgetting about the tranny gears.


That should be a sticky for any rear end discussion.

My old Dodge V10 has a 2.45 1st gear ratio, that's nearly as tall as 3rd on a 10R140! I have a 3.54 rear end. It would take a 6.23 rear end to match a modern F350 with 3.31 diffs.

Let's ask ourselves, if you bought an HD pickup 20 years ago, with a 6.23 rear end, would you honestly say: gee, this rear end is way too TALL, I'll burn up the tranny towing with it!?

A 4.30 F350 would be like by old Dodge with 8.10 diffs - heck, do they even make that? Even military Rockwells don't go that short IIRC


Deepest ratio I ever saw was 8.30 gears. If anything, the driver wished he had something deeper.

New F-750s have a 7.17 optional with the V10 and tall tires.


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Posted: 05/22/21 06:12pm Link  |  Quote  |  Print  |  Notify Moderator

John,
That 7.17 for a gas rig being able to turn some serious R's vs a diesel does not surprise. Our tires in reality are a gear into themselves. Change out from a 30" diam tire to a 32, you've dropped from a 4.10 to 3.73! If you're putting some 37-40" diam, 22"-24" rimmed tires, I can see needing 7-1 or some such LOW gearing! Or you won't get out of the hole!

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Posted: 05/22/21 08:08pm Link  |  Quote  |  Print  |  Notify Moderator

larry barnhart wrote:

When we tow up a mountain our truck wants to keep pulling but I feel better going to 4th gear and let the engine run more RPM's. not the same truck as the posts are telling about but have not had any driveline issues on any towing truck we have had. chevman

You're doing it wrong. According to a previous post, on 6% grade, you're going to need WOT and hold 7th gear and grenade the transmission. I guess 10R140 won't downshift when it needs a different ration, but a 3.55 doesn't have this issue.
/sarc

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Posted: 05/23/21 10:59am Link  |  Quote  |  Print  |  Notify Moderator

blt2ski wrote:

John,
That 7.17 for a gas rig being able to turn some serious R's vs a diesel does not surprise. Our tires in reality are a gear into themselves. Change out from a 30" diam tire to a 32, you've dropped from a 4.10 to 3.73! If you're putting some 37-40" diam, 22"-24" rimmed tires, I can see needing 7-1 or some such LOW gearing! Or you won't get out of the hole!

Marty


It's not actually that deep...final drive is about 4.10 with the OD. 65MPH would be ~2200RPM. That is with ~40" (11R22.5) tires.

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Posted: 05/23/21 11:21am Link  |  Quote  |  Print  |  Notify Moderator

philh wrote:

larry barnhart wrote:

When we tow up a mountain our truck wants to keep pulling but I feel better going to 4th gear and let the engine run more RPM's. not the same truck as the posts are telling about but have not had any driveline issues on any towing truck we have had. chevman

You're doing it wrong. According to a previous post, on 6% grade, you're going to need WOT and hold 7th gear and grenade the transmission. I guess 10R140 won't downshift when it needs a different ration, but a 3.55 doesn't have this issue.
/sarc


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Posted: 05/23/21 11:23am Link  |  Quote  |  Print  |  Notify Moderator

larry barnhart wrote:

When we tow up a mountain our truck wants to keep pulling but I feel better going to 4th gear and let the engine run more RPM's. not the same truck as the posts are telling about but have not had any driveline issues on any towing truck we have had. chevman


Yes but it is the same premise and youre absolutely doing the right thing, despite the seemingly antagonistic comments by one person. Don’t let this discussion influence how you drive. You’re tracking in the right direction.


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Posted: 05/23/21 01:42pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

Is this driver error? There are arguments for both sides, but I, like the OP, choose to change the gears myself based on what I am pulling. However, there are many out there that don't and just let computer do all the work. These are the people will generally have transmission clutch issues later on.


I'd call that design error - the input side of the transmission should always be sized for the max tq output of the engine.

Here're some charts I made a while back, 45 mph grade @ near full throttle. The 3.31 truck does it in 4th, the 4.30 truck does it in 6th:

[image]

[image]

The tq input with both gears are the same. In fact, the 3.31 is a bit lower, purely a fluke of course. At another speed the 4.30 might be a hair lower.

Point is, rear end ratio makes zero difference in RV application. In a commercial under-speed case, it may be difference because they're purposely lugging the engine down to 1100 rpm cruise. Whereas we're agreeing to lock out 9th/10th so both trucks cruise the same.

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Posted: 05/24/21 05:23am Link  |  Quote  |  Print  |  Notify Moderator

larry barnhart wrote:

When we tow up a mountain our truck wants to keep pulling but I feel better going to 4th gear and let the engine run more RPM's. not the same truck as the posts are telling about but have not had any driveline issues on any towing truck we have had. chevman


GM gives you the ability to lock out 5th gear for the reason you are using it. However, I always felt the computer did a pretty good job of selecting the appropriate gear pulling grades with my Duramax. Going down hill was a little different. In order to get any degree of engine braking with my ‘03 Duramax I needed to be in 1st gear listening to the engine rev to 5000 rpm.

In case you’re interested 5th gear with your Duramax /Allison falls between my 7th and 8th gear in my Ford with a 3.31 rear axle and your 4th is about equivalent to my 6th.

* This post was edited 05/24/21 05:43am by 4x4ord *


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