An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[brian42]

What is this MDS yall are talking about?

Multi-Displacement System (MDS) for the HEMI.

When you meet the conditions half of the cylinders "turn off" so you're basically cruising in 4-cylinder mode for better fuel economy. Plenty of heated debates about it floating around.

Pentastars (V-6) and EcoDiesels need not apply.

 

[HSKR R/T]

87 octane. I’m about 50/50 highway and city. 30 isn’t my average. It’s just what I see at 55 mph with the cruise set and mds kicked in.

That's not even remotely accurate. That's like saying you get 99mpg because that's what your dash tells you when going down a 6% grade.

 

[parnine]

What is this MDS yall are talking about?

From Wikipedia, the free encyclopedia ...... Chrysler's Multi-Displacement System (MDS) is an automobile engine variable displacement technology. It debuted in 2006 on the 5.7 L modern Hemi V8. Like Mercedes-Benz's Active Cylinder Control, General Motors' Active Fuel Management, and Honda's Variable Cylinder Management, it deactivates four of the V8's cylinders when the throttle is closed or at steady speeds.

 

[Scram1500]

MDS is like urinating on a forest fire

 

[theblet]

That's not even remotely accurate. That's like saying you get 99mpg because that's what your dash tells you when going down a 6% grade.

I'm just going off of what the instant mpg is in these conditions, which doesn't happen often.

 

[neeginan]

I am running a 3.67. By getting the 3.92 and reducing it with larger diameter tires.

Tire Size Change Gear Ratio Calculator | Interco Tire​

Calculate the proper gear ratio for a tire size change.

www.intercotire.com

Assuming stock tire size of 275 55 R20 (or another ~32 in tire):

If you have a 3.92 and go with 275 65 R20 tires, you will reduce your 3.92 to effectively a ~ 3.67. That's what I currently have.

If you have a 3.92 and go with 275 70 R20 tires, you will reduce your 3.92 to effectively a ~3.55.

If you start with the 3.21, larger tires will quickly have you in the upper 2's to ~3.0 as far as effective gear ratio goes.

 

[JD300WM]

No way to know if it's true, but the dealer I bought my 2019 from said nobody had ever said, "Must have heated front seats (cloth) and 3.92 rear." Heated seats for my wife, ratio for me. No particular reason, as I'd rarely if ever tow ... I just prefer the drive and responsiveness. We old-school types remember 3 and 4 speed transmissions, where selecting gears like 4.11 or 4.56 was cool around town, but miserable on highways. No longer an issue!

Tell your dealer that you finally found that person because I’m that guy. I wanted 3:92 gears for me and heated cloth seats for my wife and kids

 

[neeginan]

Tell your dealer that you finally found that person because I’m that guy. I wanted 3:92 gears for me and heated cloth seats for my wife and kids

I am in this club as well! 3.92, heated cloth seats and heated steering wheel.

 

[parnine]

I'm in the club and I think lots of others also as this is a common config (at least when I was shopping, there were lots of Big Horns with 3.92 and the heated seats/wheel on the lots).

 

[firsttimeramer]

So If I go from a 32" tires to a 35" tire with the 3.92 gears will I get better gas mileage ?

 

[silver billet]

Probably not; even though you lower your gear ratio with the taller tires, you will be adding a significant amount of weight and that will make things worse for MPG.

 

[theblet]

The rotational mass increase will negate the ratio change on that size of tire

 

[matab14]

Probably not; even though you lower your gear ratio with the taller tires, you will be adding a significant amount of weight and that will make things worse for MPG.

So If I go from a 32" tires to a 35" tire with the 3.92 gears will I get better gas mileage ?

I have 3.92 's with 35" tires and avg somewhere around 17mpg on highway....Winter is worse yet around 15ish.

 

[Kaderast]

This was an incredible discussion and helped me a lot with deciding on which gears I wanted when I ordered my Bighorn.

Long story short for me: the 3.21s coupled with the 5.7 engine and 8sp transmission has a crap ton of power. I usually only tow a snowmobile trailer, of small utility trailer, so the 8K pound tow rating was very adequate. MPGs have so far been very good (17-19).

Thanks for all the great discussion in this thread. Seems like 90%+ of the Bighorns on dealer lots in my area had the 3.92s. Felt like I was going out on a limb sticking with the 3.21s. It was a wise decision.

 

[theblet]

Nice. I hauled a 16 ft 1/4 inch angle iron heavy duty trailer today loaded with galvanized material. Had plenty of get up and go with the 3.21 gears. Also instant mpg was between 15 and 17. I’ll take that

 

[BowDown]

This was an incredible discussion and helped me a lot with deciding on which gears I wanted when I ordered my Bighorn.

Long story short for me: the 3.21s coupled with the 5.7 engine and 8sp transmission has a crap ton of power. I usually only tow a snowmobile trailer, of small utility trailer, so the 8K pound tow rating was very adequate. MPGs have so far been very good (17-19).

Thanks for all the great discussion in this thread. Seems like 90%+ of the Bighorns on dealer lots in my area had the 3.92s. Felt like I was going out on a limb sticking with the 3.21s. It was a wise decision.

Same here, every truck I found was a 3.92, and I wanted 3.21. Glad I waited and found the gear I wanted in the trim I wanted.
On a bad week, I average mid 17's in 80/20 city highway. On a good week, its 18 and its been consistently this way since day one

 

[brian42]

Same here, every truck I found was a 3.92, and I wanted 3.21. Glad I waited and found the gear I wanted in the trim I wanted.
One a bad week, I average mid 17's in 80/20 city highway. On a good week, its 18 and its been consistently this way since day one

I was the opposite. I wanted a 3.92 rear but the only truck I found that I liked had a 3.21.

I was skeptical until I drove it and don't regret my decision at all. Love my truck, and the 3.21 coupled with the 5.7L and ZF 8-speed is more than I need. Plenty of get-up-and-go for me and I'm very happy with it.

 

[bgalakazam]

I also wanted heated cloth seats and 3.92. Working on a custom order right now.

Tell your dealer that you finally found that person because I’m that guy. I wanted 3:92 gears for me and heated cloth seats for my wife and kids

 

[Rossi396]

Heated cloth seats with 3.92 gears...

Add me to the list! I didn’t order my truck but wanted this combination and found it at a dealer location in another nearby state.

I pull a bass boat with my truck and I drove a 3.21 and 3.92 back-to-back. I preferred the 3.92.

 

[Mountain Whiskey]

I also wanted heated cloth seats and 3.92. Working on a custom order right now.

Yep, I went the order route too. Figured I should get what I want. Didn't take long. I got 3.92 knowing I was going to lift it and put 35s on. Extra wind drag and wheel weight.

Well and because 3.92 is a far superior gear ratio.

I just had to throw that in for the fanatics.