- Joined
- Sep 25, 2019
- Posts
- 2,622
- Reaction score
- 2,335
- Location
- Rochester, NY
- Ram Year
- 2019
- Engine
- Hemi 5.7
One thing here aren't the engine RPM's higher on 3.92 verses 3.21 through the 8-speed shifting higher RPMs consume more fuel in my book.
Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.
One thing here aren't the engine RPM's higher on 3.92 verses 3.21 through the 8-speed shifting higher RPMs consume more fuel in my book.
Also, better on the valve train!!!But the 3.21 on the highway in 8th gear, it's running 400-ish rpms lower than the 3.21, and that will add up quite a bit over the life of the truck.
You can point at your speed chart all you want but I've seen multiple times people stating the the lower gear ratio will tow better. This has been with multiple different powertrains where the only difference was the rear axle ratio is different (engine, trans, body, etc. all the same). Discussions have turned into arguments over the subject with ones pointing at their charts and the others going by real world experience. The chart is great for showing RPMs but it doesn't factor in applied forces.See the chart below. If both trucks are 6th gear at 65 mph, then the 3.21 will be at 2191 rpms and the 3.92 will be at 2675 rpms.
You can point at your speed chart all you want but I've seen multiple times people stating the the lower gear ratio will tow better. This has been with multiple different powertrains where the only difference was the rear axle ratio is different (engine, trans, body, etc. all the same). Discussions have turned into arguments over the subject with ones pointing at their charts and the others going by real world experience. The chart is great for showing RPMs but it doesn't factor in applied forces.
Put this into the thought process, changing the rear axle ratio changes the amount of force applied over the rate of feed. As the gear ratio is lowered the amount of torque goes up over a shorter distance. Keep in mind towing is a constantly changing dynamic load.
Put this into the thought process, changing the rear axle ratio changes the amount of force applied over the rate of feed.
Fully agree. The 3.21 rear end is worth almost a mpg on the road over a 3.55 and more over a 3.92. Quieter and still plenty of beans. Tow/haul is there for a reason. 8 speed is there for a reason. I believe anything over 5k ought to be behind a medium duty like a 2500. Much safer.I tow a traile
i would not want to pull any travel trailer without a2500. I would rather be overly qualified then slightly under qualified
i have both the 3.92 is faster off the line but for every day driving3.21 is better. Honestly at 70 mph couple hundred rpm difference just run it in tow haul and in 7 gear
I must respectfully disagree. Towing anything at 70 MPH, that's trouble right there.I'm assuming it's a 20-foot trailer by its name. That truck will pull that trailer 70mph all day long with no trouble. No need for a 2500 or 3.92.
Reviving an old thread as I was researching the pros and cons of having a 3.21 gear vs 3.92. Several members on the forum have said it only makes a difference accelerating from a dead stop or cruising above 65mph, as the 8spd trans will choose the appropriate gear for the job at hand at any given speed during cruising. For example, at 40mph a truck with a 3.92 will cruise in 8th gear, where the same truck same scenario but with a 3.21 will be in 7th, and both trucks will be at the same rpm using the nearly the exact same effective gear ratio.
While it’s true that both trucks are at the same rpm and same gear ratio, just a different transmission gear, the similarities end there.
A truck with a 3.92 can tow better and accelerate harder at any speed vs the 3.21. Here’s a couple of reasons why:
Accelerating from 0-40 mph, a 3.92 geared truck will use all 8 gears, while a 3.21 geared truck will only use 7.
Each gear in a 3.21 geared truck is effectively longer because of the rear gear. Using only 7 gears to get from 0-40 mph means the truck spends more time in each gear. Longer time in each gear means longer time for your engine to sweep through the rpm range. While your engine is sweeping through the lower rpm range, it’s not at an rpm that makes as much power as when it’s sweeping through the higher rpm range. With a 3.92, every gear is effectively shorter, allowing the rpm’s to climb faster and get your motor to a higher rpm faster, where it can make more power.
The other reason a 3.92 is better for towing and accelerating is because it uses all 8 gears more often, as they are shorter and more ratios allows your engine to be at a more efficient rpm range more often and in more scenarios. This is one of the reasons why manufacturers aren’t selling trucks with 4 spd automatics anymore. Even if 4th gear of old trucks had the same gear ratio as our 8th gear today, it would need to be much longer. A 4spd trans would need very long gears to have its 4th gear match the ratio of todays 8th gear, causing huge rpm drops with each up shift, putting the engine at a mechanical disadvantage during lower rpms, which is inefficient. This is also why GM and Ford developed 10spd autos. More gears gives the engine more opportunity to operate at more efficient rpms more often.
Anyone who thinks a 3.92 vs a 3.21 have the same accelerating and towing capabilities once you’re moving I think is just incorrect.
Now for fuel efficiency, once you’re on the highway and your 3.21 geared
truck can use 8th gear and hold it, it will save gas over the 3.92. Lower rpm means less friction and pumping losses in the engine, saving fuel.
But there is one variable I’m not sure about: MDS. Won’t your engine be able to stay in 4cyl mode more often with a 3.92 vs a 3.21, which could help the 3.92 gear match the fuel economy of 3.21 and even beat it it in some scenarios?
Can anyone who’s had both rear gears chime in on how MDS was affected by it?
Thanks!
Another guy who doesn't understand final drive ratio's,but he did make me laugh with his theories,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.
Another guy who doesn't understand final drive ratio's,but he did make me laugh with his theories,
I think I understand them pretty well, however my experience with gear ratios are not scientific, but are real world. I've certainly ridden 15spd bicycles and could feel the difference in resistance on my legs using different gears. I've owned a 5.0 Mustang and swapped the gear from a 3.08 to a 3.73 and certainly felt the difference. In town, there was no difference in MPG, I just used top gear in the transmission more often. On the highway however was another story. I have a KTM 500 where I sometimes change rear sprocket ratios in order to facilitate different types of riding (tight woods vs fire roads for example), and I certainly feel the difference there as well.
I want to get the 3.21 on my new truck, but I think the 3.92 is the way to go for my purposes, even though I rarely tow.
Two RAM 1500 Hemi trucks with the ZF8 speed optioned exactly the same, except 1 has a 3.21, the other 3.92. Here's the claims I'm making:
There is no measurable fuel economy difference at any speed below 60mph.
The 3.92 truck will win a 0-60 race consistently.
The 3.92 will be more responsive on a roll, and will be quicker all around (roll race, from a dig, 40-70mph, whatever)
The only advantage the 3.21 has over the 3.92 is highway cruise speeds, where the motor will turn at a lower RPM in 8th gear and save fuel.
If I never speed on the highway and stick to the speed limits, I think the fuel economy difference will be next to nothing, immeasurable.
And here's one hope I have, but not sure on:
The 3.92 will allow the Hemi to run in MDS mode more often and in more scenarios, saving fuel and easily matching the economy of the 3.21 in almost all cases.
I would appreciated being proven wrong, as I will learn something new.
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.Buy a truck with a 3.21. Take out 8th gear, push the remaining 7 up, and add a very low first gear. That's essentially what you do to create a 3.92. You still have 8 gears, but the very deep first gear in the 3.92 has no effect anymore once you shift out of it.
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.As for racing, you'll have to ask Wild One but I believe a number of guys do well with the 3.21 because it doesn't need an extra shift to hit 60 mph.
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.The 3.21 also has an advantage while towing if you tow in 6th gear like I do because that is "direct drive", and at that load/speed/rpms the 3.92 has to be in overdrive/7th to match it. If you want to tow on the freeway in direct drive with the 3.92 you'll be at 2700+ rpms for no reason.
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.MDS doesn't save you any fuel in the first place. Doubling nothing is still nothing. I've disabled it from day one. However MDS cannot overcome the savings from running 400 rpms lower. All the high MPG reports come from 3.21s.
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.I couldn't care less what people prefer, if you like the 3.92 for one reason or another then that's the gear for you. But facts are facts, that's all I'm trying to argue are the facts.
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.