Gearing (3.21 vs 3.92)

[Ckamz572]

Hey, guys.

I’m sure this topic has been discussed many times on this forum, but I will ask anyway.

I currently have 2017 1500 5.7 HEMI with the 8 speed tranny. Prior to getting work done on my truck [275/65/r20 KO2s (about 34x11), Bilstein 6112s set to 2” UCAS, endlinks and 5100s in the rear], my knowledge on anything trucks was very limited. I ended up finding out I have the 3.21 gears, and when looking into a possible re-gear job, I was looking at a $2-3k+ cost!!! How big of a problem, long-term (If any problem at all) would the 3.21s pose with bigger tires? I do not tow and primarily use this truck as my daily driver with the occasional light of roading (driving to fishing holes, etc.).

 

[Ohio5pt7]

The 8 speed with the 3.21 has more mechanical advantage than a 6 speed with 3.92s. The only thing you'll one itll be slower and 2 you wont hold 8th as well as you used to.

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[NH RAM]

If you're talking about the 34's, I didn't have any issues with 34's and 3.21 gears, even while towing. The shift points changed, I lost a little pep, and my fuel economy took a hit but that was it. If you're going beyond 34's I'd consider at least correcting the speedo if you haven't already to correct the shift points; IIRC it was off by about 3% going from the factory 33" to the 34" tires. I wouldn't go beyond 35's, which is probably going to be a 34.5" tire anyway, before regearing, but that's my preference.

 

[Ckamz572]

Thanks for the replies! I have no intentions of ever going above a 35” tire. I was maybe looking into doing a 4”-6” lift down the road. I don’t care too much about acceleration (I’m driving a truck! ). I was just worried about long term damage to my transmission from the excess weight from the bigger tires and 3.21 gears.

 

[RedSRT4Me]

Thanks for the replies! I have no intentions of ever going above a 35” tire.

That's what I said till I bought 37's after my 35's

 

[Graygoose]

Ive had two trucks with 35's and 3.21'.....dont regret it. Not worth $2k IMHO

 

[RedSRT4Me]

I'm telling you now 37's definitely hurt some feelings



37's on left. 35's on right.

 

[ram1500rsm]

You'll be fine with the 8 speed and 35's you won't be forcing your trans with those tires though you'll need to correct the speedo and that in turn will address proper shift points and mpg calculations. I have the 6 speed trans with 3.92's in my RAM and i drove and offroaded 37's for almost 15k miles until couple of weeks ago when i made the switch back to 35's. The truck feels like it picked 50hp and i'm getting 2-3mpg back. You have a lot better gearing in the 8 speed trans even with 3.21's than the 6 speed with 3.92's so don't let 35's scare you. Also if you wanted to be picky with the 8th gear upshifts you could always play with it via tuning, granted that's also another monster on its own

 

[dexter]

Thanks for the replies! I have no intentions of ever going above a 35” tire. I was maybe looking into doing a 4”-6” lift down the road. I don’t care too much about acceleration (I’m driving a truck! ). I was just worried about long term damage to my transmission from the excess weight from the bigger tires and 3.21 gears.

https://auto.howstuffworks.com/auto...wing/maneuvers/gear-ratio-tire-size-chart.htm

Generally you should see higher RPM in average driving.

You should watch your oil and trans fluid to see if they need to be changed more often then with factory tires.

 

[Ohio5pt7]

https://auto.howstuffworks.com/auto...wing/maneuvers/gear-ratio-tire-size-chart.htm

Generally you should see higher RPM in average driving.

You should watch your oil and trans fluid to see if they need to be changed more often then with factory tires.

Stated that backward you would have lower rpm.

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[CheechDogg.0n37s]

I had the 3.21s with 37"s for almost a year and it was fine .... Yes I regeared because I do have 37"s but with 34" I wouldn't even worry about it at all .... Not even with 35"s for that matter ... Don't waste your money on a regear my friend you are going to be A OH K !

By the way ... my MPGs with the 37s on 3.21s as between 14-15 almost the same as with the 4.88s right now lol ...

 

[dexter]

Stated that backward you would have lower rpm.

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correct

 

[Gary2]

34.09" tires 3.92 w/8sp ,speedometer corrected and I average 16.5 around town . I would not be concerned now but I would make it a point to buy my next truck with 3.92s . Most of my previous 1500. Rams had 3.92s and one with 4.11 and one dog with 3.55s. If I could have 4.11s as an option I would of had that . Just like cubic inches , no substitute for right gears. Only reason I ended up with 3.55s was because it used to be if a 1500 Ram had 20" wheels it had 3.92s so on my 14 I saw the rotten 20s and took for granted it had 3.92s. that wont happen again

 

[RedSRT4Me]

You'll be fine with the 8 speed and 35's you won't be forcing your trans with those tires though you'll need to correct the speedo and that in turn will address proper shift points and mpg calculations. I have the 6 speed trans with 3.92's in my RAM and i drove and offroaded 37's for almost 15k miles until couple of weeks ago when i made the switch back to 35's. The truck feels like it picked 50hp and i'm getting 2-3mpg back. You have a lot better gearing in the 8 speed trans even with 3.21's than the 6 speed with 3.92's so don't let 35's scare you. Also if you wanted to be picky with the 8th gear upshifts you could always play with it via tuning, granted that's also another monster on its own

Some great point's! Going to the smaller tire will definitely give some power back both from the weight savings assuming you didn't purchase 80+ lb 35's and the effective gear ratio change.

This explanation is simplified but swapping to a smaller tire has a similar effect to keeping the bigger tire and changing the gears.

Again not exact math 37's > 35's = 3.92 < 4.56

Swapping the gears in your truck is like taking the red pill from The Matrix.

I had to see how far the rabbit hole goes and now that I've experienced gear ratio changes I could never drive a 3.21 geared truck. I mean no offense to 3.21 owners I just know myself and I wouldn't be happy till 4.10 - 4.88's are in my trucks to suit my driving preference of fast AF

 

[ram1500rsm]

Some great point's! Going to the smaller tire will definitely give some power back both from the weight savings assuming you didn't purchase 80+ lb 35's and the effective gear ratio change.

This explanation is simplified but swapping to a smaller tire has a similar effect to keeping the bigger tire and changing the gears.

Again not exact math 37's > 35's = 3.92 < 4.56

Swapping the gears in your truck is like taking the red pill from The Matrix.

I had to see how far the rabbit hole goes and now that I've experienced gear ratio changes I could never drive a 3.21 geared truck. I mean no offense to 3.21 owners I just know myself and I wouldn't be happy till 4.10 - 4.88's are in my trucks to suit my driving preference of fast AF

Agree 100%. I would gladly do 37's again if i could regear my truck and given the 6 speed trans i have i'll do a min of 4.88's or 5.13's. You guys have it better with the 8 speed trans.

 

[CheechDogg.0n37s]

I love my 4.88s lol... that's actually why I only get 13.5-14 MPGs right now because man when you put your foot down that SOB moves! lol ....

 

[tnctx02]

Hey, guys.

I’m sure this topic has been discussed many times on this forum, but I will ask anyway.

I currently have 2017 1500 5.7 HEMI with the 8 speed tranny. Prior to getting work done on my truck [275/65/r20 KO2s (about 34x11), Bilstein 6112s set to 2” UCAS, endlinks and 5100s in the rear], my knowledge on anything trucks was very limited. I ended up finding out I have the 3.21 gears, and when looking into a possible re-gear job, I was looking at a $2-3k+ cost!!! How big of a problem, long-term (If any problem at all) would the 3.21s pose with bigger tires? I do not tow and primarily use this truck as my daily driver with the occasional light of roading (driving to fishing holes, etc.).

I got the 3.21 gears with 35s and I did lose about 4 mpg. And it does not stay in 8th much anymore. That gear will never wear out on my truck now!

 

[CheechDogg.0n37s]

I got the 3.21 gears with 35s and I did lose about 4 mpg. And it does not stay in 8th much anymore. That gear will never wear out on my truck now!

It really depends on how we drive our trucks.... I almost got 17 mps this past week but I was driving at or below speed limits in town which is 45 mph lol ....

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[Flryder]

Go to grimmjeeper.com and you can look at the numbers for all kinds of gearing/tire combos and compare it to stock.

 

[rwhjr]

I have 35s with 8speed/321 and have no complaints.

Never bought it to drag race since it’s a truck and I never bought it to tow 30,000lbs since it’s a half ton.

1500s are pretty much built like an suv anyways so I treat mine as such. Rides around empty 95% of the time and when I “load it up” it’s usually just 500-1000 in the bed at most.....and with the above described setup it handles that perfectly with tow/haul mode activated.

So no complaints from me and I’m sure as hell not spending thousands to regear a 4x4. I think the 8 speed makes up for a lot....so if I had an older 6 speed maybe I’d be complaining and want stock size tires back on. But it’s not, and even when stuff is in the bed it has no problem doing stuff like merging onto the highway etc.