Axle Ratio

JerryETX said:

This is false! The 6.4 Hemi in a 2500 has considerably more towing power than its little brother. I have a good friend who pulls roughly a 10k lbs moving trailer frequently with his Ram 2500 5.7 Hemi. I have also pulled his trailer with him riding in my truck. We both agreed there’s a considerable difference between the two engines in terms of towing power. Now in terms of get up and go power yeah but the post is about towing power.

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Torque is torque, it doesn't have much more than the 5.7 at all. 5.7 = 410 lb/ft, 6.4 = 429 lb/ft.

Perhaps the torque curves will show more under the curve for the 6.4, but based on peak numbers the 6.4 is kinda poky. It's not designed to build lots of power, its designed to build power reliably all day long.

ramffml said:

Torque is torque, it doesn't have much more than the 5.7 at all. 5.7 = 410 lb/ft, 6.4 = 429 lb/ft.

Perhaps the torque curves will show more under the curve for the 6.4, but based on peak numbers the 6.4 is kinda poky. It's not designed to build lots of power, its designed to build power reliably all day long.

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You can looks at internet numbers all day long. I’m just telling you what our real world experience was.

JerryETX said:

You can looks at internet numbers all day long. I’m just telling you what our real world experience was.

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Fair enough, and you can tell me what you think you felt, I'm just telling you the "internet numbers" published by FCA.

I can tell you that the very biggest factor in towing is weather. Air flow and drag are very real. My trailer is for hauling snowmobiles (4 of them). Enclosed v-nose. If weather is clear and calm (almost never in winter), towing is noticeably easier. Also, just towing in the winter is noticeably more difficult - cold air is more dense than warm air. You can feel the drag (and I can see MPG improvement in the summer). We hauled 2 Harleys to Maine last September. I got 10 mpg on the way up. Normally get 8-9 mpg hauling the sleds in the winter. Weight difference would be a few hundred pounds.
Someone mentioned cruise control - man, that's never worked for me... way too much shifting.
FWIW. I can tell you the Ram is the king thus far in towing my trailer. Chevy's, GMC, Ford, and Toyota all get lower MPG. These were 1/2 and 3/4 ton trucks - diesels got better MPG but cost more in the long run. GMC 2500 was the worst (6.5 mpg) followed by the Tundra (7.5 mpg).

gwilburn said:

I can tell you that the very biggest factor in towing is weather. Air flow and drag are very real. My trailer is for hauling snowmobiles (4 of them). Enclosed v-nose. If weather is clear and calm (almost never in winter), towing is noticeably easier. Also, just towing in the winter is noticeably more difficult - cold air is more dense than warm air. You can feel the drag (and I can see MPG improvement in the summer). We hauled 2 Harleys to Maine last September. I got 10 mpg on the way up. Normally get 8-9 mpg hauling the sleds in the winter. Weight difference would be a few hundred pounds.
Someone mentioned cruise control - man, that's never worked for me... way too much shifting.
FWIW. I can tell you the Ram is the king thus far in towing my trailer. Chevy's, GMC, Ford, and Toyota all get lower MPG. These were 1/2 and 3/4 ton trucks - diesels got better MPG but cost more in the long run. GMC 2500 was the worst (6.5 mpg) followed by the Tundra (7.5 mpg).

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I get it. But my 2500 6.4 pulls snowmobiles in the winter and a travel trailer in the summer. I can tell you there is a world of difference. Compared to a V-nose sled trailer, my travel trailer is like pulling a huge parachute behind me. But I do agree that weather; particularly headwinds are a huge factor. As far as MPGs go, I think aerodynamics count more than weight. EXAMPLE: I had a GMC Sierra 1500 6.2L pulling 6000# trailer and got about 8 MPG and now I've got a Ram 2500 6.4 and a 7600# trailer and still get around 8 MPG.

ASPLR said:

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

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I was in a similar situation. 5.7 with a 3.21. Not a good towing gear. I was also uncomfortable with the high revs. I ended up getting rid of my 2020 Longhorn which I absolutely loved (just like you love yours) and got a 2500 (GMC) with a diesel. HD trucks were pretty hard to find last year. Over 50 vehicles in my life and this is the first diesel. It’s like night and day how the diesel tows compared to a gas. So much torque. As long as I will be pulling a camper it will always be pulled by a diesel. Anyway, my post started quite a long thread:

ASPLR said:

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 todon'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

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I have about the same camper and tow it with my 2017, hemi with 3.21. Have very little problem towing it. I was a previous truck driver and hauled tractors, so I feel I'm comfortable towing. For my truck, I have found the sweet spot to be between 60 and 65. If I push it above that the fuel economy drops very quickly.
I feel the 8 speed works very well with the 3.21 gears. I like the ride of the truck too much to consider upping to a bigger rig that will only take more fuel...

Yes, would have best to buy 2500
Gas. But 4500 lbs is full in range of
1500 , the problem is the 3.21,

3.21 are help car manufacturers
Get EPA rating an useless for
But get bag of groceries,

So need get some lower gear in the
Axle, best if going tow put 3.92
Engine will run about 10% fast in the right gear all time. This kill freeways
Mpg by about 1.5 mpg,
Your city drivers will be about same or maybe 0.5 mpg better,
Everyday driver,

Tow this 3.92 will pull lot better
More driverble tow lot nicer.
EPA makes engine spin up
Under load for emissions
So 4000 rpm in normal for all engines,
Hopefully this helps,
Willie,

Ok so what is a normal transmission temperature range at 65 for the 8 speed. I have a 1500 Laramie eco diesel and it runs about 190-200 depending on speed

OP you could have your truck regeared for considerably less than buying a new truck. Usually runs $500-1K per axle. Check a few 4x4 shops for cost. If you don't have a LSD it is also a good time to add a Detroit TrueTrac.

If you added oversize tires regearing can actually increase the unloaded fuel mileage as it would get the truck back to the designed RPM/MPH speed range. Not a big a difference with the 8-10 speeds used these days but was could be 3-4mpg difference back in the 4 speed days.

Willie Mosher said:

Engine will run about 10% fast in the right gear all time. This kill freeways

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This is incorrect. Your RPMs will not change. You need to calculate (final) gear ratios for both trucks, and then notice how close they are. Example:

3.21 in 6th gear = 3.21 * 1 = 3.21
3.92 in 7th gear = 3.92 * 0.82 = 3.2144

Your engine will not change in the slightest, you just end up always one numerical gear ahead of the 3.21 once both trucks are beyond second.

The differences are from a dead stop, and when the 3.21 hits 8th gear. Other than that, no real difference.

Here is a good video explaining why 3.92 is not giving you any more power on the highway than the 3.21
https://www.youtube.com/watch?v=lLGPHl1o0bc

The common thinking is that 3.92 gives you more wheel torque in each transmission gear, which is true. But the flipside to that coin is that it also gives you higher engine RPMS, meaning you need to now upshift sooner into the next gear - which just eliminates the torque advantage you had, because a 3.21 in 6th gives identical wheel torque as a 3.92 in 7th. Similar things happen with 5th and 6th and 4th and 5th.

In fact, whats even more interesting, some of these gears actually benefit the 3.21 over the 3.92. Most of the time when the 3.21 is in 7th, the 3.92 will be in 8th, but when those two trucks are in those gears the 3.21 will have more torque to the ground: 2.63 vs 2.5.

Think about that. The 3.21 in 7th will pull harder than the 3.92 in 8th, RPMs will be slightly higher in the 3.21 as well due to that gearing.

In other words: your wife asks you to go the market to pick up 12 apples. You have two choices, Vendor A will sell you 4 baskets with 3 apples each, and Vendor B will sell you 3 baskets with 4 apples each.

Does it matter which one you pick? Nope, multiplying 4 x 3, or 3 x 4 gives you the same answer: 12.

That's how it works with gears. You have a transmission gear ratio and an axle gear ratio, but what matters is multiplying those 2 numbers together to get your final gear ratio. And when you do that, you'll find that both trucks have very very close final gear ratios to choose from.

Only from a dead stop does the 3.92 pull harder, that's because the 3.21 can't downshift any further to make up for the deep rear axle gear. But out on the highway, the transmission compensates for the taller axle ratio and will give you very similar final gear ratios.

ASPLR said:

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

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Is the new trailer higher or wider or brakes dragging, etc.
IF it is higher and wider than the old trailer, that's what you are feeling more than the weight.

Aero drag is really noticeable on my 3.92 equipped 1500 also - in the summer I can see a mpg loss at sunset on vs. during the day - just due to the humidity change and cooler air running empty.

ramffml said:

This is incorrect. Your RPMs will not change. You need to calculate (final) gear ratios for both trucks, and then notice how close they are. Example:

3.21 in 6th gear = 3.21 * 1 = 3.21
3.92 in 7th gear = 3.92 * 0.82 = 3.2144

Your engine will not change in the slightest, you just end up always one numerical gear ahead of the 3.21 once both trucks are beyond second.

The differences are from a dead stop, and when the 3.21 hits 8th gear. Other than that, no real difference.

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If this is the case why do all of the truck manufacturers of trucks sold in America increase towing capacity with only lowering the rear axle ratio? For example Ram 2500 with a 4:10 rear axle ratio has more listed towing capacity than a 3:73 rear axle ratio.

Or are you just saying the rpm’s doesn’t matter?

JerryETX said:

If this is the case why do all of the truck manufacturers of trucks sold in America increase towing capacity with only lowering the rear axle ratio? For example Ram 2500 with a 4:10 rear axle ratio has more listed towing capacity than a 3:73 rear axle ratio.

Or are you just saying the rpm’s doesn’t matter?

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Because of the extra torque that's being absorbed by the tranny... That's why.
Pulling like I do would be way harder on a 3.21 than on my 3.92.

If the argument held water, I could put bigger tires on and get better fuel mileage... not going to happen.

JerryETX said:

If this is the case why do all of the truck manufacturers of trucks sold in America increase towing capacity with only lowering the rear axle ratio? For example Ram 2500 with a 4:10 rear axle ratio has more listed towing capacity than a 3:73 rear axle ratio.

Or are you just saying the rpm’s doesn’t matter?

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Because of the first gear. When the 3.21 is in third, it can downshift yet to second, or first. But when both trucks are first, in that scenario obviously the 3.92 is going to have a major advantage.

So those are the differences; when both trucks are in first, or when both trucks are in 8th, but in between both trucks will use their transmissions to get the proper final gear ratio needed.

gwilburn said:

Pulling like I do would be way harder on a 3.21 than on my 3.92.

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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).

Right - it's the TOTAL gear ratio reduction from the flywheel to the rear axle that matters to the engine.

Need to take into account the transmission gear ratios and the fixed axle gear ratio. Then you have the torque increase across the transmission gear range.

HEMIMANN said:

Right - it's the TOTAL gear ratio reduction from the flywheel to the rear axle that matters to the engine.

Need to take into account the transmission gear ratios and the fixed axle gear ratio. Then you have the torque increase across the transmission gear range.

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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?

lpennock said:

OP you could have your truck regeared for considerably less than buying a new truck. Usually runs $500-1K per axle. Check a few 4x4 shops for cost. If you don't have a LSD it is also a good time to add a Detroit TrueTrac.

If you added oversize tires regearing can actually increase the unloaded fuel mileage as it would get the truck back to the designed RPM/MPH speed range. Not a big a difference with the 8-10 speeds used these days but was could be 3-4mpg difference back in the 4 speed days.

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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.