What's new
By:
5thGenRams Forums

Register a free account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members through your own private inbox!

3.21 vs 3.92

techieanalyst said:
I said my opinion when comparing 3.92 in the 5.7 and 5.0 ford, this wasnt to say its slow :)
Click to expand...

Did you have the 3.92 gears put in your 5.0 aftermarket? I've only seen 3.73 as the biggest numerical gear ratio. 3.31, 3.55, and 3.73 are all I show ever available from the factory from 2015+.
Now, since you mentioned quickness, the 3.92 geared 4x4 hemi 2019's are 0-60 in the 6.6 second range. The 5.0 Ford is nearly 1.5 seconds quicker to 60.
 
I hope I don’t regret going with the 3.21. Having a bit of buyers remorse. I do a bit of towing and driving in the mountains...
 
filup01 said:
I disagree with most of the people here. I have 3.21 and average 17-17.7 MPG per tank. I drive normal, not slow but not like I’m 17 y/o either. I also live in Atlanta where traffic is hell and small portions of my commute is bumper to bumper but mostly it’s wide open interstate. I’ll settle for saying my 30 mile each way to work is a mix of city and highway.
Nonetheless, when I set the cruise at 75-80 driving to work on the interstate drinking my coffee and listening to a morning show on the radio, I see the transmission go back and forth between 7th and 8th gear on the small hills down here. Mind you, the Hemi has more than enough power to easily make it up a hill on a paved interstate at low elevation. My thoughts are that the 3.21 gears are so tall that at that speed with high wind drag and a gentle slope, the 3.92 gears wouldn’t downshift and thus, may possibly get better gas mileage than the 3.21.
This is just my theory anyway. I haven’t lifted and have stock tires on my 4X4 (Offroad package) and am 100% happy with my truck. I have another truck in my driveway right now; a 2001 Dodge 2500 4X4 Cummins. It gets marginally better fuel economy but with diesel being $1.00 at the pump per gallon and more expensive oil and fuel filter changes, I’ll drive the new gasoline truck with 3.21 gears and just drink my coffee in peace and quite while the truck downshifts. Plus, the old truck, although almost mint condition with exception to having 200k miles, is very stiff. A piece of gravel will bounce you so high that when you land, you’ll snap your spine. The new Ram is so very comfortable no matter the gears. It is a truck and not a Prius. I think it matters what you like regardless of gears. For anyone buying a new truck, today I don’t think you could do better than a Ram and I’ve recently owned all makes.
Click to expand...
Here’s another way to look at this. Ask yourself why your 3.21 truck is down shifting on those modest hills. It’s not that the truck can’t maintain your speed in 8th gear. The Hemi certainly could. It’s downshifting because under that higher load condition 7th gear will maintain that speed more efficiently. The truck is providing you the best mpg for that scenario. But once you’ve crested the hill the truck shifts back to 8th since 8th is more efficient in that lower load situation. Modern vehicles have more gears than those of the past in order to keep the engine at the most efficient rpm across a wide range of conditions.

Now since the 3.21 in 7th is a nearly identical final ratio as the 3.92 in 8th, any time the 3.21 is in 8th gear, because the computer has determined that is the most efficient gear for the task, it has the advantage over the 3.92 mpg wise. The 3.92 would need a ninth gear to match the efficiency of the 3.21 at those low load situations, and it’s just not there.
 
I'm curious how you come to the math in those numbers. The diff ratio difference is 3.21 to 3.92, yet the difference between 7th & 8th gear in the transmission is 0.84 to 0.67.
 
lambone said:
I hope I don’t regret going with the 3.21. Having a bit of buyers remorse. I do a bit of towing and driving in the mountains...
Click to expand...
How much weight you towing, how far, how often? If it’s more than 7000-7500lbs I’d get the 3.92 for sure. Less than that I think you’ll be fine. The 8 speed is amazing. It will put you in the right gear to get you up those mountains and the Hemi has plenty of power. I just wouldn’t want to get too close to the max tow rating for the 3.21. The transmission is worked harder with the 3.21.
 
Gman said:
I'm curious how you come to the math in those numbers. The diff ratio difference is 3.21 to 3.92, yet the difference between 7th & 8th gear in the transmission is 0.84 to 0.67.
Click to expand...
3.21 to 3.92 is about a 20% difference .
.67 to .84 again 20% difference. I did the actual calculation once. It was almost spot-on.
 
Zeronet said:
3.21 to 3.92 is about a 20% difference .
.67 to .84 again 20% difference. I did the actual calculation once. It was almost spot-on.
Click to expand...

When I use a calculator like this one: https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

...interchanging the 3.21 with a transmission gear of 0.84 (7th) at 2000 RPM gives me a speed of 70.42.

...interchanging the 3.92 with a transmission gear of 0.67 (8th) at 2000 RPM gives me a speed of 72.3.

Close, but not the same.

And for comparison, turning the 3.21 in 8th at 2000 RPM gives a speed of 88.29. The 3.92 can't gear any higher. That's about a 16 MPH difference in 8th gear at the same RPM.

ETA: This is all based on the truck having tires with a diameter of 31.9" (OEM Bridgestone H/L Alenza 275/55R20 on my Limited)
 
Last edited:
Gman said:
When I use a calculator like this one: https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

...interchanging the 3.21 with a transmission gear of 0.84 (7th) at 2000 RPM gives me a speed of 70.42.

...interchanging the 3.92 with a transmission gear of 0.67 (8th) at 2000 RPM gives me a speed of 72.3.

Close, but not the same.

And for comparison, turning the 3.21 in 8th at 2000 RPM gives a speed of 88.29. The 3.92 can't gear any higher. That's about a 16 MPH difference in 8th gear at the same RPM.

ETA: This is all based on the truck having tires with a diameter of 31.9" (OEM Bridgestone H/L Alenza 275/55R20 on my Limited)
Click to expand...
Never said they were EXACTLY the same. I did say nearly, almost. You’re splitting hairs but yes, it’s about 50 rpm difference between a 3.92 truck in 8th and a 3.21 in 7th at 70mph. I call that pretty darn close.
 
On other mpg threads it has been shown that the 3.92 has a 2.5-3.5 mpg penalty over the 3.21 at speeds above 65. For those living in areas that have good 4-lane highways and commute 30 or more miles, the 3.21 should be considered. Those stating there is little or no difference apparently don't drive at higher speeds daily.
 
I have 3.21 and I have a button on dash for towing. Only tried it with Snowmobile trailer but seems to work.
 
Last edited:
Guys, just compare the gears side-by-side on http://www.grimmjeeper.com/gears.html and come to your own conclusions.

3.21 in 7th is close to (actually pulls a little bit better than) 3.92 in 8th. So when your 3.21 truck shifts back up into 8th you're running where the 3.92 truck WISHES it had a 9th gear.

Finally, 1450 rpm @ 65 mph has long been considered the sweet-spot for fuel economy in diesel engines. Not sure how this translates to gas engines, but guess which gear gives you ~1450 rpm @ 65 mph?
 
ExcursionDiesel said:
On other mpg threads it has been shown that the 3.92 has a 2.5-3.5 mpg penalty over the 3.21 at speeds above 65. For those living in areas that have good 4-lane highways and commute 30 or more miles, the 3.21 should be considered. Those stating there is little or no difference apparently don't drive at higher speeds daily.
Click to expand...

I have the 3.21s and I just put 33s on my truck. I commute 30 miles each way on the freeway and am getting between 19.1 and 17.5 on the hwy. In town I am getting between 16.7 and 14.5. My combined tank has been around 16-17. And it still tows my 5500 lb boat/trailer just fine.
 
SOME MATH: The final reduction for the 3.92 in 8th is 2.6532. The final reduction for the 3.21 in 7th is 2.6964. The difference is .0432 or 1.62% - essentially a wash. My approach is that if you do only occasional towing with a heavy load, and don't have plans to increase tire diameter, or have some other particular reason to choose the 3.92, you could do your towing duties just fine by limiting shifts to 7th gear. Then the remainder of the time you could have the benefit of the 3.21 in 18.1% lower engine rpm when in 8th. I don't believe it is inconvenient to drive with a 7-speed transmission. Your last truck probably had fewer gears than that.
 
You must log in or register to reply here.

Users who are viewing this thread

Total: 1 (members: 0, guests: 1)

Site Vendors

https://www.jasonlewisautomotive.com/

Members online

Total: 626 (members: 31, guests: 595)
Back
Top