How critical is axle ratio?

[Hemling]

In the modern day of 10 speed automatics, how necessary are 'towing' gears? By 'towing' I mean anything over 4.xx. These new transmissions have such low first gears compared to just a decade ago, is it really necessary to get deep gears anymore? I understand it makes getting going easier on all the moving parts, if you have a 4.30 compared to say a 3.55 or something, but doesn't it all come out in the wash? What I mean is, a truck with lower numeric gears will just tow in a lower gear, while a truck with deeper gears will allow the transmission to hold a higher gear. Is there a tradeoff or 'sweet spot' that anyone can speak to? I am still towing with an Excursion, so everything is a trade off. I could install 4.56 gears, but only have 4 gears. I could keep 3.73 gears, but know I'll always be in 3rd or 2nd to pull. I know that the towing guides always list bigger weights the deeper the gears get, but with a modern 10 speed automatic, I'm just not seeing the big deal anymore.

[blt2ski]

Issue with just going by axle ratio to get your GCWR per X brand engineers. Is you in reality may be wanting a different speed - 35-40mph depending upon gvwr of tow rig, max start grade - 12%, to name two specs in the current jsae tow spec.
I personally want closer to 30% or greater in 1st gear, and my states min speed of 40mph on a freeway grade. Speed is HP spec, grade starts imity is overall low gear plus a tire radius factor times torque. So if you go strictly by GCWR manufactures give you, your drivetrain may or may not be enough for your needs a d wants.
My 02 on the disCUSSion.

Marty

[Crespro]

4x4ord wrote:
I think choosing the best gear ratio begins with having a good understanding of your engine’s powerband. Seeing a torque curve of the 7.3 would be very useful. A bsfc map would also be helpful. After getting a sense of your engine’s capabilities knowing the transmission’s gear ratios is essential. Finally knowing how the truck will be used needs to be considered.
I don’t know much about the 7.3 other than it has a long flat torque curve and I believe it makes 430 peak hp at 5500 rpm. It makes 425 lb ft of torque at 3250 rpm (263 hp). and 350 hp at 3900 rpm.

350 hp should be more than enough power to tow a 12000 lb rv up a 6% grade at 60 mph.

With 3.55 gears and the 10 speed 7 th gear would put you at 2150 rpm at 60 mph with 34 inch tires and 4th gear would put the engine at 3800 rpm at 60 mph.

Without giving it much more thought I’d agree with Shiners earlier post:
If your going to be running empty a fair bit get 3.55 gears and you’ll have the opportunity to get better fuel economy unloaded on the highway. While towing you can lock out the top gears and still have all the performance you need, seldom dropping below 4th gear in the hills.

If your primarily going to be towing with the truck you may as well get 4.30 gears to gain a little bit of grunt at the rear wheels to get your trailer moving in first gear. The odd time when your running empty down the highway you’ll be at 2060 rpm in 10 th gear at 75 mph…. which doesn’t sound too bad.

4x4ORD has good insights. With my truck and 15K fifth wheel, the 3.55 Ford engineer limits of 14.8K and 21.8K are too low. The 4.30 has Ford limits of 18.9K and 26K. These provide a reasonable comfort margin (about 25% and 15%). IMHO, the OP would be happier in the long run with greater towing margin.

[4x4ord]

I think choosing the best gear ratio begins with having a good understanding of your engine’s powerband. Seeing a torque curve of the 7.3 would be very useful. A bsfc map would also be helpful. After getting a sense of your engine’s capabilities knowing the transmission’s gear ratios is essential. Finally knowing how the truck will be used needs to be considered.
I don’t know much about the 7.3 other than it has a long flat torque curve and I believe it makes 430 peak hp at 5500 rpm. It makes 425 lb ft of torque at 3250 rpm (263 hp). and 350 hp at 3900 rpm.

350 hp should be more than enough power to tow a 12000 lb rv up a 6% grade at 60 mph.

With 3.55 gears and the 10 speed 7 th gear would put you at 2150 rpm at 60 mph with 34 inch tires and 4th gear would put the engine at 3800 rpm at 60 mph.

Without giving it much more thought I’d agree with Shiners earlier post:
If your going to be running empty a fair bit get 3.55 gears and you’ll have the opportunity to get better fuel economy unloaded on the highway. While towing you can lock out the top gears and still have all the performance you need, seldom dropping below 4th gear in the hills.

If your primarily going to be towing with the truck you may as well get 4.30 gears to gain a little bit of grunt at the rear wheels to get your trailer moving in first gear. The odd time when your running empty down the highway you’ll be at 2060 rpm in 10 th gear at 75 mph…. which doesn’t sound too bad.

[kellem]

Hemling wrote:

kellem wrote:

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?!?

Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.

So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?

When not towing 9th and 10th engage as normal when speed Dictates.....the higher gears are all influenced by speed.

[BenK]

Hemling wrote:

snip...

So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?

to answer this, you will need to provide some contextual info...like your engine torque/HP curve, what truck/trailer/etc, terrain and how fast you wish to accelerate and constant speed you will maintain in the terrain you wish to travel

Close ratio tranny's provides more gear ratios to keep your engine at the optimum RPMs at whatever speed you are trying to maintain.

[Grit_dog]

Hemling wrote:

kellem wrote:


Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.

So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?

They could be used downhill or with a stiff tail wind. Otherwise 9 & 10 are not useful or efficient gears for any sizeable trailer towing.
Especially with high gears like 3.55s and the bigger diameter tires that are common on newer trucks.

[Hemling]

kellem wrote:

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?!?

Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.

So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?

[Terryallan]

deltabravo wrote:

Terryallan wrote:
So if you have one of the makes we made axles for..

Who is this "We" you speak of?

When I started. It was GETRAG GEAR. A German company. Then years later, we were sold to Dana Axle. a few years later, GETRAG bought us back. Until finally we were sold to GKN Driveline.

and recently there was a hostile take over of GKN, by Melrose. We were then to be sold again. But as of now. No buyer can be found. And it is still called GKN.

[kellem]

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?!?

Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.

[Terryallan]

Huntindog wrote:

Terryallan wrote:

Huntindog wrote:

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.

Actually that is not true. I made ring, and pinion gears for 23 years. each company has different specs for the steel used in their gear sets. There are a infinite number of metal combinations, and each one has a different heat treat spec. How long it is heated, how hot, and how it is quenched.
It is true that one supplier, supplies many different manufacturers, and each manufacturer has different quality demands. Different metal combinations, and different tolerances for quality.

Dana Axle makes the majority of American truck gear sets, and axles. Or they did 2 years ago. GKN Driveline makes axles for SUVs, and vans, and some passenger cars. They supply BMW, Ford, GM, Fiat/ Jeep/Chrysler, and Maserati. And yes, some are weaker, and some are stronger than others. it all depends on the application. after all the axles in a Chevy Equinox, don't need to be as strong as the axle in a Maserati.

Fun fact. Many ring gears are no longer bolted on. They are welded. However you still have to change both the ring, and pinion if you ever need to replace one or the other. They are married together through a process called Lapping.. During which they are run together with a abrasive compound spraying on them, under high torque to "wear" them together, so that they will run quietly. Then they are put into a testing machine to see if they indeed make noise. If they pass they go to the next step, if they fail. they are scrapped, and sent back to be melted down.
Then when they are put into the axle, they are tested for noise again. It is a very interesting process, well at least for the first year.
So if you have one of the makes we made axles for.. and it moves when you press the gas. You can thank.... me. LOL

BTW. Each gearset has 2 sides. A Drive side, and a Coast side. and they are exactly what they say. But in general. One side is always harder to get right. If your vehicle is quite going down the road, but roars when you let off the gas. You have a problem on the coast side.

So do you think there is an appreciable difference in the HD truck axles among the big 3?

It doesn't really matter in this conversation if a Maseratti has stronger axles than a Yugo. I would expect that to be the case.

I merely answered your statement that all manufacturers used the same steel, and all have the same quality . Short answer is. they do not. There is a vast amount of difference in the steel used by each company. Some steel is brittle, and some isn't. some have to be shotpeened to relax the steel so it doesn't break. It isn't all the same.
Could it be that one company or the other used a more brittle steel for a while? Yes it is entirely possible. Ford had a recall on the ring, and pinions in the 2004 F150s, because of bad steel.

[deltabravo]

Terryallan wrote:
So if you have one of the makes we made axles for..

Who is this "We" you speak of?

[ShinerBock]

Lynnmor wrote:

valhalla360 wrote:
If you are within the trucks ratings, talk of increased stress is a red herring.

The manufacturers have designed to account for it.

Is that why all trucks last forever? 😄

Yeah! Like the current half-tons will last the same as an HD with the 14k tow ratings they have these days. Right...

valhalla360 wrote:
If you are within the trucks ratings, talk of increased stress is a red herring.

The manufacturers have designed to account for it.

Not really. Even Dana-Spicer, one of the largest driveline manufacturers in the world, states that the life expectancy of driveline components decreases with lower axle ratios.





The above images are from research done by Dana-Spicer on their driveline products in the heavy-duty truck market where trucks typically cruise at around 1,400 rpm. Keep in mind that this is using their product well below the truck's ratings and only using 200 hp. Eaton and Meritor did the same with their transmissions and clutches, but I do not have the link.

Many times, speccing a lower axle ratio to save money on fuel economy is negated by added maintenance/replacement cost of the parts that this lower axle ratio effect. If one does not tow close to their ratings that often then it is probably better to get a lower axle ratio. However, if you are towing closer to your max ratings on a regular basis then the higher gear option is probably the best way to go for longer component life.

[Lynnmor]

valhalla360 wrote:
If you are within the trucks ratings, talk of increased stress is a red herring.

The manufacturers have designed to account for it.

Is that why all trucks last forever? 😄

[valhalla360]

If you are within the trucks ratings, talk of increased stress is a red herring.

The manufacturers have designed to account for it.