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 > How critical is axle ratio?

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ShinerBock

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Posted: 06/15/21 09:28am Link  |  Quote  |  Print  |  Notify Moderator

The need for a low or high number rear gear ratio boils down to how the truck is being used and how often. If it is unloaded most of the time and you occasionally tow closer to its max ratings then it is probably best to get a lower number rear gear ratio and just select lower trans gears as needed when towing. If it is towing most of the time then it is probably best to get a higher rear gear ratio.

Getting a lower rear gear ratio to get better fuel economy on a truck that is towing 75% of the time makes no sense just as getting a higher gear ratio for a truck that is unloaded 75% of the time it is used.


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Groover

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Posted: 06/15/21 09:34am Link  |  Quote  |  Print  |  Notify Moderator

Interestingly, in prior years to get the highest tow rating on the F150 you had to order the 3.55 gears instead of the 3.73. In 2019 it was 13,200lbs for the 3.55 gears vs 11,700lbs for the 3.73 gears.
For 2021 the 3.55 and 3.73 are tied at 14,000 max.

While looking that up I noticed that the 2022 Ford Towing guide is out though not yet complete.

Here is the link for those interested

2022 Ford towing guide

Hemling

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Posted: 06/15/21 10:09am Link  |  Quote  |  Print  |  Notify Moderator

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?

ShinerBock

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Posted: 06/15/21 11:54am Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


This will tell you what rpm each rear gear will be at certain speeds.

RPM Calculator

Grit dog

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Posted: 06/15/21 12:52pm Link  |  Quote  |  Print  |  Notify Moderator

dodge guy wrote:

Huntindog wrote:

Groover wrote:

Bottom line is that drive shaft RPM is irrelevant. What really matters is whether the engine is in its power band when you need it to be. Back when truck transmissions only had a 2.5 to 1 spread between high and low axle rations where much more important than they are with the newer transmissions that have a 7 or 8 to 1 spread.

The op is correct, unless you find yourself in first gear wishing that you had a deeper gear or in 10th gear wishing that you had a higher gear you get the same effect by simply changing gears.

I do suspect though that axles built with higher ratios may also be built to handle higher torque to the wheels, at least in some cases.
We have a winner. Rather than write all of this again, here is the readers digest version.
GM was late to the party of increasing their HD lines tow ratings. Ford and Ram were first, and they required higher (numericially) ratios to do it. When GM tried this approach, the Pinion gears would not live behind the Dmax. For those that do not know: as the rear end gears increase numericially, the pinion gear engages less of the ring gear, and thus is weaker.
So though the Dmax was more than capable of a higher tow rating, the gear set to make the performance acceptable was out of reach.
Enter GMs new Allison 10 speed. With lower gears available in the tranny, the rear gearset could actually be changed in the direction that would make it stronger. So that is what they did, and for good measure they increased the size of the ring gear on the duallys as well.


What I get out of that is that GM’s axles are weak. No other manuf had that issue with lower gears.


LOL. What exactly makes an AAM 11.5 axle under a Chebbie weaker than the same axle under a Dodge?


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Huntindog

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Posted: 06/15/21 01:40pm Link  |  Quote  |  Print  |  Notify Moderator

dodge guy wrote:

Huntindog wrote:

Groover wrote:

Bottom line is that drive shaft RPM is irrelevant. What really matters is whether the engine is in its power band when you need it to be. Back when truck transmissions only had a 2.5 to 1 spread between high and low axle rations where much more important than they are with the newer transmissions that have a 7 or 8 to 1 spread.

The op is correct, unless you find yourself in first gear wishing that you had a deeper gear or in 10th gear wishing that you had a higher gear you get the same effect by simply changing gears.

I do suspect though that axles built with higher ratios may also be built to handle higher torque to the wheels, at least in some cases.
We have a winner. Rather than write all of this again, here is the readers digest version.
GM was late to the party of increasing their HD lines tow ratings. Ford and Ram were first, and they required higher (numericially) ratios to do it. When GM tried this approach, the Pinion gears would not live behind the Dmax. For those that do not know: as the rear end gears increase numericially, the pinion gear engages less of the ring gear, and thus is weaker.
So though the Dmax was more than capable of a higher tow rating, the gear set to make the performance acceptable was out of reach.
Enter GMs new Allison 10 speed. With lower gears available in the tranny, the rear gearset could actually be changed in the direction that would make it stronger. So that is what they did, and for good measure they increased the size of the ring gear on the duallys as well.


What I get out of that is that GM’s axles are weak. No other manuf had that issue with lower gears.
That is one possibility. But it is unlikely....I do not know who the axle suppliers are for the different brands and for what years, but I have read that for some years/models the same supplier sells to more than one manufacturer,,, even so, I doubt that one brand of axle is much stronger than another. A 11" inch ring gear is the same size in any brand, and the steel will be the same as well.
What is more likely is the Dmax actually puts out more power for longer.

There is a lot of evidence to back this up over the years.... If one chooses to look at it.



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valhalla360

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Posted: 06/15/21 01:49pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

The need for a low or high number rear gear ratio boils down to how the truck is being used and how often. If it is unloaded most of the time and you occasionally tow closer to its max ratings then it is probably best to get a lower number rear gear ratio and just select lower trans gears as needed when towing. If it is towing most of the time then it is probably best to get a higher rear gear ratio.

Getting a lower rear gear ratio to get better fuel economy on a truck that is towing 75% of the time makes no sense just as getting a higher gear ratio for a truck that is unloaded 75% of the time it is used.


Back in the days of 3 or 4 speed transmissions, you really had to choose what you wanted to optimize (towing or not towing) when selecting the rear end ratio.

This is the beauty of the 8/10 speed transmissions. Even up near the tow limits, the engine can stay in it's ideal RPM range by selecting an appropriate transmission gear.

Loaded or empty, if you stay within ratings, the overall gear ratio is good with the lower (numerical) gear ratio.

The only time you need to consider using a higher (numerical) gear ratio, is if you are over the tow rating of the lower gear ratio.


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valhalla360

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Posted: 06/15/21 01:55pm Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


What's likely to happen with the 4.3 is you will run a gear or two lower and the RPM will be the same when towing. Where you will turn extra RPM is when you are running empty, hit 10th gear and the RPM keeps climbing.

With the 3.73, the worst 5th wheel tow rating is 16k (depends on cab, box and 4x4 configuration), so 12-13k is well under it's capability.

ShinerBock

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Posted: 06/15/21 02:39pm Link  |  Quote  |  Print  |  Notify Moderator

valhalla360 wrote:

ShinerBock wrote:

The need for a low or high number rear gear ratio boils down to how the truck is being used and how often. If it is unloaded most of the time and you occasionally tow closer to its max ratings then it is probably best to get a lower number rear gear ratio and just select lower trans gears as needed when towing. If it is towing most of the time then it is probably best to get a higher rear gear ratio.

Getting a lower rear gear ratio to get better fuel economy on a truck that is towing 75% of the time makes no sense just as getting a higher gear ratio for a truck that is unloaded 75% of the time it is used.


Back in the days of 3 or 4 speed transmissions, you really had to choose what you wanted to optimize (towing or not towing) when selecting the rear end ratio.

This is the beauty of the 8/10 speed transmissions. Even up near the tow limits, the engine can stay in it's ideal RPM range by selecting an appropriate transmission gear.

Loaded or empty, if you stay within ratings, the overall gear ratio is good with the lower (numerical) gear ratio.

The only time you need to consider using a higher (numerical) gear ratio, is if you are over the tow rating of the lower gear ratio.


I have to disagree here. The more frequent you tow closer to your upper limits, the better you are in getting a higher numerical gear ratio to keep added driveline stresses low that a lower gear ratio brings.

It is similar to the half-ton versus HD debate. Towing close to a half-ton's 10k rating on a rare occasion does not warrant getting an HD, but if you do it on a regular basis then you are better off getting an HD because the added stress will cause premature wear on the half-ton as to where an HD is built to handle it.

Crespro

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Posted: 06/15/21 04:27pm Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


I run the 4.30 with the 7.3 gas engine. About 2,000 rpm in tenth gear at 65 mph empty. It pulls a 15K fifth wheel fine, but of course, you know you are loaded. The TFL test was a 16K gooseneck trailer up the IKE, so I knew this would be adequate power. I would get the 4.30 with the 10 speed -- excellent combo. I am rated for 26K, and run at 22.5K GCWR.


Crespro 2021 Grand Design 310GK-R, 2020 F250LB, 7.3L, 4.30, Reese 27K

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