I said my opinion when comparing 3.92 in the 5.7 and 5.0 ford, this wasnt to say its slow![]()
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.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.
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.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...
3.21 to 3.92 is about a 20% difference .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.
How much weight you towing, how far, how often.
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.
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.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)
I'm not sure why you're getting defensive. I never said you said they were "exactly the same". I'm just trying to get as much understanding about this subject as I can.Never said they were EXACTLY the same.
I’m good, I find this an interesting subject as well.I'm not sure why you're getting defensive. I never said you said they were "exactly the same". I'm just trying to get as much understanding about this subject as I can.
I feel like one guy said it right, if you drive unloaded 80-90% of the time 3.21 is good.I found this 16 page thread to be informative and helped reassure that I won’t regret having the 3.21.
https://www.ramforum.com/threads/3-21-gear-ratio-anyone-actually-towing-with-it.43001/
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.