Axle Ratio

[lpennock]

Re-gearing the front end of a 2012 and newer 4X4 1500 entails buying a completely new front diff.You can't change gears in the ZF front diffs as the carrier is welded ,and nobody sells a servicable gear set for them.

Wow that's a dumb design. That would be a problem. Glad I have a 3500 with a real axle up front.

BTW: The compete 3.92 diff is $1375 from mopar parts Giant.

68349705AA - Genuine Mopar Axle-Service Front

MoparPartsGiant.com offers the great deal for genuine Mopar parts, 68349705AA. Axle-Service Front for $1375.00. All parts are backed by the Mopar's warranty.

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[HEMIMANN]

What has more torque at the wheel; a 3.21 in 5th or a 3.92 in 7th?
What has more torque at the wheel; a 3.21 in 6th or a 3.92 in 7th?

It would be additive, right? Add the gear ratios then multiply the engine torque at the rpm per the trans gear selected. Then you can compare wheel torque curves.

In my younger days I would graph in Excel, but spring arrived this week after 5 long months of bad winter and I'm swamped with spring chores, so have at.it.

 

[Wild one]

The 8hp75 is also (IIRC) used on the Ram trx which puts out 650 lb/ft of torque vs the 410 lb/ft of my 5.7. I am not worried about my transmission in the slightest.

Nobody pulling 8000 pounds needs to worry about the 8hp75, and half tons won't really pull more than that anyway regardless of gear ratio because of other limits to the truck (payload, suspension, light duty truck etc).

The TRX uses the 8HP95 which is rated at just over 700 lb-ft of torque compared to the 8HP75 which is rated just over 553 lb-ft of torque,while the 8HP70 is rated at 516 lb-ft of torque. Those are crank torque ratings,not wheel torque ratings.
The TRX /TrackHawk both use the heavier duty 8HP95,which is rated at more torque then the 8HP90's used in the Hellcat cars,which is rated at 663 lb-ft.

The last 2 digits of the ZF transmission are it's torque rating in newton/meters multiplied by 10.

8HP70 = 700 nm = 516 lb-ft
8HP75 = 750 nm = 553 lb-ft
8HP90 = 900 nm = 663 lb-ft
8HP95 = 950 nm = 700 lb-ft

 

[ramffml]

It would be additive, right? Add the gear ratios then multiply the engine torque at the rpm per the trans gear selected. Then you can compare wheel torque curves.

Let's assume we have two otherwise identical trucks with one 3.21 and one 3.92. They're both pulling the same 6000 lb TT at 65 mph on straight ground with no real wind. Tire size is 32 inches.

Here are the calculated final gear ratios for the trucks, as well as the calculated RPMs they'd be at depending on which gear they're in:



Notice the blue cells. At 65 mph, the 3.21 could be in 6th gear at 2191 RPMs, and the 3.92 in 7th gear would be at 2194 RPMs. (I picked the details in this scenario because I know my truck and my trailer and what gear it would be in at 65 mph on a flat road with little wind, but the argument holds regardless of what the variables are, as long as the variables affect both trucks equally.)

I don't know what the 5.7 hemi itself is putting out at those RPMs, but it doesn't matter because it would be the same for both trucks because both trucks are within 3 RPMs of eachother. Meaning, while the 3.21 is in 6th gear, and the 3.92 is in 7th, if they're both at the same MPH and both at the same RPMs, they'd both be putting the same torque at the wheels.

If the 5.7 is at the same RPMs in both trucks at the same speed, it must be putting out the same torque to the wheels in both trucks.

Ok so now both trucks hit a steep grade and can no longer maintain speed. The 3.21 drops to third before it finally revs high enough to produce enough torque from the hemi. What gear would the 3.92 most likely be in? Gear 4 is 4602 RPMs which only differs form the 3.21 by 86 RPMs. It's the most likely gear, because if it was in 3rd it would be redlining at 5725 and if it was in 5th it probably can't put down the required power at those rpms - if the 3.92 COULD be at 3505, why would the 3.21 be at 4688 when it can be in 4th gear at 3768 RPMs and still be putting more power down vs the 3.92 at 3505 RPMs?


So again, we know the 3.92 puts down more power in the same numerical gear number. But this is only HALF the story; the other half of the story is that the 3.21 doesn't HAVE to be in the same numerical gear. It can, and will be in one numerically lower gear at pretty much the same engine RPMs; and when the RPMs are the same and the final gear ratio is the same, then both trucks are putting the same torque to the wheel.

 

[Wild one]

Techinically if the 3,21's were in 6th,it would be putting down more wheel torque then the 3.92's in 7th.
6th is 1.00:1 in the 8 speeds which is direct drive through the transmission,while 7th is overdrive and is going through a planetary.
That's why you always dyno an 8 speed in 6th.
It won't really make any differance in the long run,but 6th is the most efficient gear in the 8 speeds.

 

[HEMIMANN]

Are we saying (without going through all the math, but there's enough above) it makes sense with transmissions with lot of gear rations (8, 9, 10) to use the lowest axle gear ration and let the tranny take care of the rest?

i.e. - it will upshift when dead-heading for best mileage, and downshift when loaded where it's happy.

 

[ramffml]

Are we saying (without going through all the math, but there's enough above) it makes sense with transmissions with lot of gear rations (8, 9, 10) to use the lowest axle gear ration and let the tranny take care of the rest?

i.e. - it will upshift when dead-heading for best mileage, and downshift when loaded where it's happy.

Pretty much that's my take on it. The 3.92 is like taking the 3.21, removing 8th gear, and adding an extra granny gear. You still have 8 gears, you just slid 7 of them side ways and replaced an overdrive with an extra deep first gear.

Logically that's the best way to think of it. Once you're out of that deep first/second gear, both trucks have pretty much identical final gear ratios to choose from; and if the final gear ratios are the same, given the same speed both trucks will be putting the same torque down to the wheels.

This ignores (like @Wild one already mentioned) other little issues like transmission gear efficiency's, not every gear is equally efficient, but I'm not sure we need to get that detailed though it may be interesting to do the math at some point. It's like saying I way 20 pounds less than you therefore my identical truck will accelerate quicker than yours. Technically true, but not significantly important IMHO.

 

[lpennock]

Are we saying (without going through all the math, but there's enough above) it makes sense with transmissions with lot of gear rations (8, 9, 10) to use the lowest axle gear ration and let the tranny take care of the rest?

i.e. - it will upshift when dead-heading for best mileage, and downshift when loaded where it's happy.

To a point. IMO it really doesn't make any sense to have such a tall axle ratio that the only time the trans can get into the highest gear is when going down hill unloaded with a tail wind.

 

[ramffml]

Techinically if the 3,21's were in 6th,it would be putting down more wheel torque then the 3.92's in 7th.
6th is 1.00:1 in the 8 speeds which is direct drive through the transmission,while 7th is overdrive and is going through a planetary.
That's why you always dyno an 8 speed in 6th.
It won't really make any differance in the long run,but 6th is the most efficient gear in the 8 speeds.

Yep agreed, and it's even more interesting: taking my downshift at the hill example when the 3.21 is in third, its RPMs are 4688 whereas the 3.92 is most likely at 4602. At that point, the 3.21 is actually putting more power down; higher rpms = higher torque for the most part (until it hits peak power but that's above 5000 RPMs IIRC).

Same thing if the 3.21 is 3700 vs the 3.92 at 3500.

 

[ramffml]

To a point. IMO it really doesn't make any sense to have such a tall axle ratio that the only time the trans can get into the highest gear is when going down hill unloaded with a tail wind.

Yes agreed, but what is that point? If you're suggesting the 3.21 can never maintain 8th gear then I'm going to vehemently disagree. My truck LIVES in 8th gear. Literally (when not towing) it spends 95% of its run time in 8th gear. I do all rural/highway driving.

Towing, yep it will never hit 8th, but that doesn't mean it's the wrong choice of rear end unless all you're ever doing is towing. If you're daily driving your truck, 3.21 makes the most sense for the vast majority of people.

 

[lpennock]

Yes agreed, but what is that point? If you're suggesting the 3.21 can never maintain 8th gear then I'm going to vehemently disagree. My truck LIVES in 8th gear. Literally (when not towing) it spends 95% of its run time in 8th gear. I do all rural/highway driving.

Towing, yep it will never hit 8th, but that doesn't mean it's the wrong choice of rear end unless all you're ever doing is towing. If you're daily driving your truck, 3.21 makes the most sense for the vast majority of people.

I'm not saying a stock truck won't maintain the speed. The issue is when oversize tires are added. Depending on the tire size that could push 8th (or 10th) into the unusable range.

 

[ramffml]

I'm not saying a stock truck won't maintain the speed. The issue is when oversize tires are added. Depending on the tire size that could push 8th (or 10th) into the unusable range.

That is definitely true. And for guys who plan to run oversized tires or do a lot of rock crawling in 4x4 low in first/second gear, the 3.92 is definitely the better choice. I've never suggested otherwise in those cases.

 

[Dusty]

I own a 2019 1500 Laramie Crew Cab. 5.7 HEMI 3.21 axle ratio. It pulled my old camper like a dream at 3050 pounds. My NEW travel trailer is 4500 pounds unloaded, dual axle. I don't load it up. I travel pretty light. The issue I have is with high RPM'S. Seems I have to crawl on the highway at 60mph or hit 3500 rpms or MORE on hills. It's not "comfortable" to me. The truck struggles. I'm thinking about trading trucks. I'm looking for a 2500 gas but I don't want to break the bank. They are hard to come by. I'm wondering if I choose a 1500 with a 5.7 and a 3.92 axle ratio. Will it help to reduce rpms? I don't want to fly down the road but sometimes I like to drive the speed limit, 68 or 70.
Much appreciated. BTW, I've owned this truck for 2.5 years and I LOVE it. The Harmon Kardon is the DEAL. This is my 4th RAM
Thank you

I've pulled my 6900 lb. trailer and tractor with two different 1500 Rams equipped with the 3.92, and now with a 3.21 axle.

If I had to do it all over again, I'd order another Ram with the 3.21 axle ratio. For the six or seven times I pull every year, all the 3.21 costs me is a little patience when getting up to speed, yet yields 5-6 more MPG when I'm not pulling.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33-gallon fuel tank, factory dual exhaust, 18” wheels. Build Date: 3 June 2018. Now at 89145 miles.

 

[U&A]

A healthy 5.7 and 6.4 hemi is happy towing between 2,500-4,000rpm all day.

run it!!