2023 5.7L V8 HEMI MDS VVT eTorque Engine 3.21 Rear Axle Ratio

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ramffml

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The 10 speed can save fuel over the 8 speed because there are times when the engine can drop 200 rpms and still carry the required load. However dropping 400 rpms might be too low and then you're lugging so the truck has to pick the higher-than-needed gear/rpms. If you have 10 gears, you have more possibility to drop RPMs. That's where the fuel savings come from, you have more ratios to choose from but more importantly they're closer together so you have more ability to drop RPMs.

The 3.21 vs 3.92 is different because at all speeds above (roughly) 15 to 20 mph you're always at the same RPMs in both trucks. That's what we're trying to tell you. Once you're off the line, at no point in the city/freeway is the 3.92 running any significant different rpms vs the 3.21. You're just using a different gear number in the transmission but your engine is still turning at the same speed. And then of course once the 3.21 hits 8th gear its game over and the 3.92 will never match the fuel savings of the 3.21 in 8th.
 

Tulecreeper

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The short answer is you're wrong, I'm way too tired of this argument to get into details about it but if you actually sit down and calculate the final gear ratio's for these 2 exact trucks you'll see why.
What you say here is true, however up to 40mph, the 3.92 truck can use all 8 gears where the 3.21 will only use 7. 8 gear ratios available to choose from is better than 7. The motor can run closer to its optimal RPM range for the task at hand when it has more gears to choose from. This is why a 10speed can save even more fuel.

Yes I didn't consider the extra shift that may be necessary. It's dependant on your trap speed and how much HP your truck is making. But in all fairness, the ZF8 shifts at lightning speed, and I doubt the extra shift would lose all the advantage you've gained from the very short 1st gear on launch with the 3.92.

So you're saying 6th gear direct drive has less driveline loss than 7th gear. Probably true. Maybe that could save more gas than a 3.92 in 7th doing the same job?? It's a pretty rare scenario when pitted against all the other potential driving situations we get into.

I don't believe MDS doesn't save fuel. It HAS to save fuel. Running in 4cyl mode during cruise takes advantage of one thing in two ways: Less pumping losses. 4 cylinders needs to work twice as hard as 8 cylinders, causing your throttle body position to be opened further, reducing intake vacuum. High intake vacuum is lost energy. MDS activates when the cylinder being turned off is at bottom dead centre, closing both valves. When that dead cylinder is on it's way back up and compressing the air but not igniting it, the pressurized air pushes the piston back down, and the cycle repeats itself. It's similar to a highly inflated basketball. If you drop it, it will bounce back up, but not quite as high as the point you dropped it from. That's the measurable loss. Keeping that cylinder running when it's not needed creates pumping losses, such as pulling air into that cylinder on every stroke, and pushing that exhausted air out the exhaust valve. All this is negated when the valves are kept closed.

I appreciate you humouring me and my thoughts. I will visit a dealer and try out the 3.92. I'm betting I'll be sold.
The first gear on the 3.92 is very short lived. You're not spending any time in that gear. Again, if there was any possibility that Ram thought the 3.92 saved fuel it would be the default/only option to purchase as there would be no point to the 3.21 then.



We all understand the theory on how this works. In practice, MDS only activates when you're throttle inputs are so low that you're basically idling (slight exaggeration but you get the point). Right before MDS activates, the truck is already using no fuel, so trying to save an additional 10% of no fuel still equals no fuel.

This fuel saving from MDS is not larger than the fuel savings you get from running at 400 rpms lower. The 3.21 even with MDS disabled saves more fuel than the 3.92 with MDS enabled.
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Wild one

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What you say here is true, however up to 40mph, the 3.92 truck can use all 8 gears where the 3.21 will only use 7. 8 gear ratios available to choose from is better than 7. The motor can run closer to its optimal RPM range for the task at hand when it has more gears to choose from. This is why a 10speed can save even more fuel.

Yes I didn't consider the extra shift that may be necessary. It's dependant on your trap speed and how much HP your truck is making. But in all fairness, the ZF8 shifts at lightning speed, and I doubt the extra shift would lose all the advantage you've gained from the very short 1st gear on launch with the 3.92.

So you're saying 6th gear direct drive has less driveline loss than 7th gear. Probably true. Maybe that could save more gas than a 3.92 in 7th doing the same job?? It's a pretty rare scenario when pitted against all the other potential driving situations we get into.

I don't believe MDS doesn't save fuel. It HAS to save fuel. Running in 4cyl mode during cruise takes advantage of one thing in two ways: Less pumping losses. 4 cylinders needs to work twice as hard as 8 cylinders, causing your throttle body position to be opened further, reducing intake vacuum. High intake vacuum is lost energy. MDS activates when the cylinder being turned off is at bottom dead centre, closing both valves. When that dead cylinder is on it's way back up and compressing the air but not igniting it, the pressurized air pushes the piston back down, and the cycle repeats itself. It's similar to a highly inflated basketball. If you drop it, it will bounce back up, but not quite as high as the point you dropped it from. That's the measurable loss. Keeping that cylinder running when it's not needed creates pumping losses, such as pulling air into that cylinder on every stroke, and pushing that exhausted air out the exhaust valve. All this is negated when the valves are kept closed.

I appreciate you humouring me and my thoughts. I will visit a dealer and try out the 3.92. I'm betting I'll be sold.
Sit down with a calculator and figure out the final drive ratio's in each gear,then you'll see how your theory leaves alot to be desired.In fact i think Ramffml has already posted them in this thread if you go looking.Even with 3.92's you're going to have to be going downhill with a very light foot to get into 8th at 40 mph,and don't lift your foot off the accelerator,as the truck will downshift to maintain speed.I don't think you comprehend how the 8 speed works,as it doesn't act like an old school 3 speed auto does. If you're towing the truck will downshift to it's rpm sweet spot anytime it needs to downshift,and that rpm will be virtually identical whether you have 3.21's or 3.92's,it'll just be in a differant transmission gear to get to that rpm.
 
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Huliodude

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The 10 speed can save fuel over the 8 speed because there are times when the engine can drop 200 rpms and still carry the required load. However dropping 400 rpms might be too low and then you're lugging so the truck has to pick the higher-than-needed gear/rpms. If you have 10 gears, you have more possibility to drop RPMs. That's where the fuel savings come from, you have more ratios to choose from but more importantly they're closer together so you have more ability to drop RPMs.
Yes I agree with this. The 10spd can also make more power at different speeds by being able to get the motor at the desired RPM range for the task at hand. There is a point of diminishing returns in number of gears vs total gear spread vs complexity of the transmission.
The 3.21 vs 3.92 is different because at all speeds above (roughly) 15 to 20 mph you're always at the same RPMs in both trucks. That's what we're trying to tell you. Once you're off the line, at no point in the city/freeway is the 3.92 running any significant different rpms vs the 3.21. You're just using a different gear number in the transmission but your engine is still turning at the same speed. And then of course once the 3.21 hits 8th gear its game over and the 3.92 will never match the fuel savings of the 3.21 in 8th.
Ok I understand now. Because we're comparing a 3.92 vs a 3.21, that effectively adds a gear to the bottom at the expense of a gear at the top, and all other gears in between are basically equal, both trucks tow equally in gears 2-8 for the 3.92 and 1-7 for the 3.21 for the most part, at least once you are moving. The 3.21 truck will just be in a lower gear than the 3.92 but the effective gears ratios are the same except for 1st & 8th. So perhaps on a roll, both trucks will feel the same and accelerate the same, will just be in a different gear.

This means off the line, the 3.92 has the short 1st gear advantage, and on the highway, the 3.21 has a fuel efficiency advantage, so long as the truck can stay in 8th gear. Anywhere in between, it's a wash.

I still believe that the 3.92 will achieve the same fuel economy as the 3.21 up to 50mph or so, but after that, the 3.21 starts saving more fuel. But you're right, if you're not towing heavy, it's not the best gear to have if you also are looking for the best all around fuel economy. Thanks for clarifying.

However if we were comparing a 3.55 vs a 3.92 or a 3.21, everything would be different. You have to admit that the 3.21 in 8th gear is an incredibly long gear ratio, and the truck will downshift on some grades to hold speed. If the hemi was available with a 3.55, I bet it would be the ultimate sweet spot for most tasks. Give up a little bit of 8th gear length to shorten first gear a bit, and all other ratios will shift a little sooner as well compared to the 3.21. No idea why RAM doesn't offer it, perhaps for CAFE requirements.
 

Tulecreeper

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However if we were comparing a 3.55 vs a 3.92 or a 3.21, everything would be different. You have to admit that the 3.21 in 8th gear is an incredibly long gear ratio, and the truck will downshift on some grades to hold speed. If the hemi was available with a 3.55, I bet it would be the ultimate sweet spot for most tasks. Give up a little bit of 8th gear length to shorten first gear a bit, and all other ratios will shift a little sooner as well compared to the 3.21. No idea why RAM doesn't offer it, perhaps for CAFE requirements.
I don't know about this model year, but they used to offer the 3.55 in 4x4 and diesel engine models.
 

Wild one

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I don't know about this model year, but they used to offer the 3.55 in 4x4 and diesel engine models.
They only offered 3.55's with the hemi and 8 speed in the very short mid year 2013 run ,after that all you can get with the Hemi/8 speed combo is 3.21 and 3.92.The RFE equipped trucks with the 5/6 speed could be had with 3.55's,and the V6 trucks with the 8 speed could be had with 3.55's,and the eco-diesal could be had with 3.55's,but not the Hemi with the 8 speed,whether they were 2X4 or 4X4.
I think they flucked up by not offering the 3.55's with the Hemi/8 speed combo after 2013,as it would of been a good middle of the road combo.That would of had slightly differant final drive ratio's compared to the 3.21/3.92 combo's which carry virtually identical final drive ratios in the middle gears when the transmission gear's are factored in
 

ramffml

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Yes I agree with this. The 10spd can also make more power at different speeds by being able to get the motor at the desired RPM range for the task at hand. There is a point of diminishing returns in number of gears vs total gear spread vs complexity of the transmission.

Ok I understand now. Because we're comparing a 3.92 vs a 3.21, that effectively adds a gear to the bottom at the expense of a gear at the top, and all other gears in between are basically equal, both trucks tow equally in gears 2-8 for the 3.92 and 1-7 for the 3.21 for the most part, at least once you are moving. The 3.21 truck will just be in a lower gear than the 3.92 but the effective gears ratios are the same except for 1st & 8th. So perhaps on a roll, both trucks will feel the same and accelerate the same, will just be in a different gear.

This means off the line, the 3.92 has the short 1st gear advantage, and on the highway, the 3.21 has a fuel efficiency advantage, so long as the truck can stay in 8th gear. Anywhere in between, it's a wash.

I still believe that the 3.92 will achieve the same fuel economy as the 3.21 up to 50mph or so, but after that, the 3.21 starts saving more fuel. But you're right, if you're not towing heavy, it's not the best gear to have if you also are looking for the best all around fuel economy. Thanks for clarifying.

However if we were comparing a 3.55 vs a 3.92 or a 3.21, everything would be different.

Exactly. 3.55 changes the equation because the RPMs are always different at the same MPH. The math for the 3.21 and 3.92 works out so beautifully that the RPMs basically don't change that much except on the extreme end.

You have to admit that the 3.21 in 8th gear is an incredibly long gear ratio, and the truck will downshift on some grades to hold speed. If the hemi was available with a 3.55, I bet it would be the ultimate sweet spot for most tasks. Give up a little bit of 8th gear length to shorten first gear a bit, and all other ratios will shift a little sooner as well compared to the 3.21. No idea why RAM doesn't offer it, perhaps for CAFE requirements.

It's an overdrive gear, and a tall one, but my truck literally lives in 8th gear. Downshifts to 7th are only 400 rpms and they are smooth and seamless. It's not the same as a 4 gear tranny where you feel every downshift and hear the thing wind up to level 10. I have no trouble holding speed on the freeway or even climbing smaller grades above 120km/h without a downshift. You're in Ontario, if you're famililar with the Skyway bridge over Hamilton/Burlington, my truck doesn't downshift climbing it. I can hold 8th unless I start trying to pass people of course.

I can run from Hamilton to Bracebridge over the 407, 400, highway 11 and only downshift like 2 or 3 times (not including passing). I've done it more than once.
 
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