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

[silver billet]

My only complaint is that it seems to kick out of 8th on even the slightest incline

If you had the 3.92 still, but this one was special and came with a 9th gear for further overdrive, would you complain when it shifts out of 9th back down to 8th? Or would you love the increase in MPG, lowered RPMs and think "I can deal with the occassional downshift back down to the 8th and enjoy the 9th where it works, that all other poor souls driving the 3.92 don't get"?

If you would take the 9th gear, congrats, you have it now with the 3.21.

That's how I look at it anyway

 

[MayhemMOORE]

If you had the 3.92 still, but this one was special and came with a 9th gear for further overdrive, would you complain when it shifts out of 9th back down to 8th? Or would you love the increase in MPG, lowered RPMs and think "I can deal with the occassional downshift back down to the 8th and enjoy the 9th where it works, that all other poor souls driving the 3.92 don't get"?

If you would take the 9th gear, congrats, you have it now with the 3.21.

That's how I look at it anyway

Valid point. Perhaps I shouldn't have labeled it so much as a complaint, and more as just a difference I have not adjusted to yet. All in all I think I will be happier long-term with the 3.21's as they fit my needs better and will still tow whatever I need the few times a year I tow anything.

 

[BowDown]

All a 3.21 truck is running in 7th is a 3.92 truck in 8th so I don't really get that complaint

 

[Rossum]

All a 3.21 truck is running in 7th is a 3.92 truck in 8th so I don't really get that complaint

I don't think any of us 3.21 people would have a complaint at all if we could lock the trans into 8th. The downshifts (especially on cruise control) aren't necessary most of the time.

I wish the + and - buttons on the steering wheel would function more like a proper manual mode. You press one the first time, it locks the trans into whatever gear it's in. From there on, it stays in that gear unless you command it up or down with a button. It would still be allowed to auto upshift it you hit the engine's redline, and auto downshift if engine drops to idle speed in whatever gear you've selected. To revert back to regular Drive mode, press both buttons at the same time.

 

[BowDown]

I don't think any of us 3.21 people would have a complaint at all if we could lock the trans into 8th. The downshifts (especially on cruise control) aren't necessary most of the time.

I wish the + and - buttons on the steering wheel would function more like a proper manual mode. You press one the first time, it locks the trans into whatever gear it's in. From there on, it stays in that gear unless you command it up or down with a button. It would still be allowed to auto upshift it you hit the engine's redline, and auto downshift if engine drops to idle speed in whatever gear you've selected. To revert back to regular Drive mode, press both buttons at the same time.

That is my only dislike, it's a bit too aggressive when cruise control is engaged. I don't need you to get back up to the set speed in 3/100ths of a second, chill.

 

[navynate88]

Ok... so basically from reading all of this (which is a bunch) I probably made the wrong decision getting the 3.92 vs 3.21 for what I will be doing which is mostly commuting. Bought the truck while I was still overseas and didn't have all the information (like the damn non-power folding tow mirrors). That being said, are there any tricks that people use to help get better MPGs with the 3.92? I drive like a respectable person most the time and I am only getting about 14.5 average which a good mix of highway and city.

 

[Scram1500]

are there any tricks that people use to help get better MPGs with the 3.92?

On the highway try to stay at 2000 rpm, which is approximately 72 mph with the 3.92. Going above 2000 rpm is when the fuel consumption really increases.

Going to a taller tire (within reason as bigger tires weigh more) will lower highway rpms as it effectively changes the gear ratio. I went to a 33" from the OEM 32" and now when I'm at 72 mph the engine is around 1800 instead of the 2000 rpm from before. I was getting 20 mpg on the lieometer at 72 mph before the winter fuel

 

[323F2R]

Thank you so much for your number punching. Would you know where to find a torque and horsepower graph of the 2020 hemi, without torque? I would like to see rpm’s when the torque starts to fall off. Wouldn’t that be the optimum shift point as long as the torque remained high in the next gear?

 

[navynate88]

On the highway try to stay at 2000 rpm, which is approximately 72 mph with the 3.92. Going above 2000 rpm is when the fuel consumption really increases.

Going to a taller tire (within reason as bigger tires weigh more) will lower highway rpms as it effectively changes the gear ratio. I went to a 33" from the OEM 32" and now when I'm at 72 mph the engine is around 1800 instead of the 2000 rpm from before. I was getting 20 mpg on the lieometer at 72 mph before the winter fuel

What do you mean by going from the stock 32" to 33"? are you talking sidewall height? I know on mine the rim size is 20". What kind of tires are you running? I am looking to replace my OEM Duelers ASAP.

 

[flyfingers]

What do you mean by going from the stock 32" to 33"? are you talking sidewall height? I know on mine the rim size is 20". What kind of tires are you running? I am looking to replace my OEM Duelers ASAP.

Total height of tire. Regardless of wheel (rim) size, OEM's are right close to 32" tall. 18" tire / wheel combo will have more tire side wall than a 20" tire / wheel combo but they will both be right close to 32" tall.

 

[Scram1500]

What do you mean by going from the stock 32" to 33"? are you talking sidewall height? I know on mine the rim size is 20". What kind of tires are you running? I am looking to replace my OEM Duelers ASAP.

I hear you there, the oem Duelers are made out of dogshit. I have 18" rims and the stock size is 275/65 r18 (32" overall diameter), I went with Michelin Defenders 275/70 r18 (33" overall diameter). With a 20" rim the stock tire is 275/55 r20 (32" diameter). For a 33" tire you'll want 275/60 r20

 

[Buz]

With a 20" rim the stock tire is 275/55 r20 (32" diameter). For a 33" tire you'll want 275/60 r20

Just remember, in addition to the $1,000 you're spending to get the bigger tires mounted and balanced, anyone doing this will also need to purchase something to recalibrate your speedometer. (Unless you're ok with an inaccurate speedometer)

 

[richardfl]

Great explanation, although it will not end the debate, as folks feel they need to defend their purchases to the death regardless of facts.
Thanks for putting the time and effort on this write-up. Should be a sticky.

Cheers,

My 3.21’s have Pixie dust and are better than you 3.92’s.

 

[nfmejia]

Great explanation, I never really understood the difference. I now I’m confident I have the right setup with 3.21, as I don’t tow but benefit from fuel economy for my many highway miles.

Off topic, but saw your signature. How's that Flowmaster American Thunder Cat-Back sound?

 

[brian42]

Ok... so basically from reading all of this (which is a bunch) I probably made the wrong decision getting the 3.92 vs 3.21 for what I will be doing which is mostly commuting. Bought the truck while I was still overseas and didn't have all the information (like the damn non-power folding tow mirrors). That being said, are there any tricks that people use to help get better MPGs with the 3.92? I drive like a respectable person most the time and I am only getting about 14.5 average which a good mix of highway and city.

Accelerate briskly (but not excessively) to cruising speed (~3000 RPM give or take). By making that one change to my driving habits I am getting about 1 mpg overall (hand-calculated).

I was getting mid-14s for mpg with my 3.21 gears and by getting up to cruising speed quickly instead of easing up to it now has me in the mid-15s for mpg overall.

 

[Kxlexus]

Perfectly happy with my 3.92 gears and wouldn’t want it equipped any other way

 

[Silent Bob]

This is such a great post. It was a big help in deciding which to choose.

 

[Borsig]

As a new member this is a great post. Reading this has me hard leaning to 3.21's. I will tow, but most likely rarely, and much less often than the turck will travel on the highway on trips.

 

[bwsRam19]

I just stepped up to 33 inch tall tires, sofa king glad I went with 3.92......sofa king glad, thanks.

 

[DTN]

When I bought my 2003 2500 5.7 hemi I was always amazed how much getup and go it had. I thought it had the 3.92 axle and it towed my 8000# boat with little effort. When I was getting ready to trade it in recently I looked at the specs again. Turn's out it has the 4.1 axle. That 2003 hemi wanted to run mid-grade too but I never thought the mpg was that bad for a truck. Now with the new 1500 5.7 hemi with the 3.92 axle I can still tow that boat and I should get better mpg ... and it takes regular less expensive gas.

Thanks for the gearing data. Very informative.