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

bigwhttrk19dt

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Newton’s Second Law dictates that the force on an object is equal to its mass multiplied by its acceleration. So, increasing your wheel size will decrease the driving force from your wheels which will culminate in a decrease in acceleration of said wheels.
To summarize, a car’s engine finds it more difficult to rotate larger wheels, making for a decrease in overall acceleration. This is all assuming that every other component like the engine, driveshafts, gears and differentials are kept stock, making them specifically engineered for the original wheel size. Fuel economy will inevitably suffer as well due to the engine having to work harder to rotate the wheels, but I’d imagine that’s the least of worries for the generic modifier.
But you already knew that.
 

hutchman

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From the speed 0-30 MPH, 3.92 has higher final drive ratio over 3.21 (18.46 vs 15.12) until it has to shift to 2nd gear at 30MPH.

A really good post with great information except.......and I hate open a can of worms, but....

A final drive ratio of 18.46 is not "higher" than 15.12. Gear ratios that are numerically greater are "lower" gears. A lower numerically ratio is a "higher or taller" gear. As in the above quite, the 18.46 is a numerically greater number, but it is a lower gear.

I see this mistake being made on more and more forums. I guess I'm just an old gearhead on a one man mission trying to correct the terminology.
 

Jack

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A really good post with great information except.......and I hate open a can of worms, but....

A final drive ratio of 18.46 is not "higher" than 15.12. Gear ratios that are numerically greater are "lower" gears. A lower numerically ratio is a "higher or taller" gear. As in the above quite, the 18.46 is a numerically greater number, but it is a lower gear.

I see this mistake being made on more and more forums. I guess I'm just an old gearhead on a one man mission trying to correct the terminology.
Correct.
 

dailyherold

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Might be a dumb question, but with this excellent data by @Jack, are there some optimal speed ranges that those of us with a 3.92 can stay in to optimize fuel efficiency? I don't really care much because the truck drives amazing, and I didn't buy it for fuel efficiency, but my commute is all surface streets with sections that can get up to 50mph max. If I knew e.g. gear 6 in low 40s was sweet spot I might accel to that a wee bit quicker and resist going beyond.

I've kind of tried experimenting a bit with acceleration this last fill up, getting to a cursing speed quicker, then cruising more with momentum, and found my truck calculated mpg is higher than normal. I'll hand calculate in the next 2 days but wondering if maybe this data could validate better efficiency with this driving habit versus slow steady accel to same cruising speed.
 
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Rob5589

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Might be a dumb question, but with this excellent data by @Jack, are there some optimal speed ranges that those of us with a 3.92 can stay in to optimize fuel efficiency? I don't really care much because the truck drives amazing, and I didn't buy it for fuel efficiency, but my commute is all surface streets with sections that can get up to 50mph max. If I knew e.g. gear 6 in low 40s was sweet spot I might accel to that a wee bit quicker and resist going beyond.

I've kind of tried experimenting a bit with acceleration this last fill up, getting to a cursing speed quicker, then cruising more with momentum, and found my truck calculated mpg is higher than normal. I'll hand calculate in the next 2 days but wondering if maybe this data could validate better efficiency with this driving habit versus slow steady accel to same cruising speed.
Just drive as you normally would. Someone else on the forum had a fantastic post. To sum up, they stopped watching their mpg and just drove and enjoyed their new Ram.
 

Adrianp89

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I specifically wanted the 3.92s. Now that I have a very fast car in the garage next to the truck, I wish I had went with the 3.21 for better economy. Either way the truck is slow as hell and I never push it lol.
 

Happiicamper

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Thank you for the time and effort for doing this!!!!


Sent from my iPhone using Tapatalk
 

jdmartin

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Just drive as you normally would. Someone else on the forum had a fantastic post. To sum up, they stopped watching their mpg and just drove and enjoyed their new Ram.
That might have been me. I'm a notorious do-not-carer when it comes to truck fuel economy. I accept that it sucks relative to virtually all other vehicles and just enjoy the ride, or pick one of my other vehicles that is more appropriate for the job. Like today, I needed to let someone in that locked themselves out of a property, and I wanted to stop and feed the ducks. Neither of which required the truck, which just came back from a 1500 mile rounder to and from Florida. :)
 

Willwork4truck

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Pretty balanced opinion, granted he likes the extra power and 35’s.

I now have more power than any other vehicle I‘ve ever owned and plan to stay stock rims and pretty much stock tires so I guess I‘m somewhat in the minority.

I’d agree on his points of a heavier truck, with taller heavier tires and running 75-80 with a headwind or even at in-town speeds likely does better with the 3.92. This is where that 3.55 set of gears would have made a lot more sense to me, but alas, FCA didn’t offer it.

It’s a never ending discussion based on different types of trucks and different wants/needs, but that’s ok, I like “first world” problems as compared to say 3rd world choices...
 

VernDiesel

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Great job Jack. What differences are their transmission wise with respects to the 2020 Ecodiesel vs like what you have shown here with the Hemi? I am sure generally the results 3.21 vs 3.92 play out the same but probably not exactly with respects to what speeds which final drive ratio has the advantage and more.

BTW I am uber impressed with this transmission. Its a big part of what has made this a generally better truck IMO than Ford/GM since 2013 when it first became available. I have an older version being in the 4th gen but 640,000 miles on it over half with a TT or boat in tow and zero issues with it. Most tows are between 5 & 7k. I do change the fluid every 100k and have changed the filter 3 times. FWIW its behind an Ecodiesel with both GDE ECM & TCM tunes. You can read about GDE's TCM tune here if so inclined. https://www.greendieselengineering....-1500-EcoDiesel-Transmission-Tune/2_51.action
 
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Jack

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Great job Jack. What differences are their transmission wise with respects to the 2020 Ecodiesel vs like what you have shown here with the Hemi? I am sure generally the results 3.21 vs 3.92 play out the same but probably not exactly with respects to what speeds which final drive ratio has the advantage and more.

BTW I am uber impressed with this transmission. Its a big part of what has made this a generally better truck IMO than Ford/GM since 2013 when it first became available. I have an older version being in the 4th gen but 640,000 miles on it over half with a TT or boat in tow and zero issues with it. Most tows are between 5 & 7k. I do change the fluid every 100k and have changed the filter 3 times. FWIW its behind an Ecodiesel with both GDE ECM & TCM tunes. You can read about GDE's TCM tune here if so inclined. https://www.greendieselengineering....-1500-EcoDiesel-Transmission-Tune/2_51.action
I do not know if the Diesel transmission has the exact same gear ratio. They should all be variations of the German ZF 8-speed transmission. It's one of the best in the industry. I am personally not a big fan of the Italian designed Eco-diesel. The last 2 generations of the engine all had problems, so I am wary of the 3rd generation even though they claim it is completely redesigned. In addition, the eco-diesel is more of a passenger diesel than a diesel designed with towing in mind like the one found in GM half ton trucks. The eco-doesel does not come with exhaust break unlike the GM 3.0 straight 6. However the GM interior and ride is just not refined enough for me, and towing is something I do once in a blue moon. I wish Ram puts a Cummins straight 6 in there for the folks who tow a lot.
 

Dusty1948

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The 3.92 should have better acceleration than 3.21, but does anyone have any idea of the time difference from 0-60? Are there any actual numbers out there?
No numbers, but I'll offer a seat-of-the-pants opinion.

Since I went to a 1500 Quad Cab DT, 5.7, 8HP75 with 3.21 axle, from a 1500 Quad Cab DS, 5.7, 3.92 axle, the difference seems to be this:
1. 3.92 launches are quicker in 1st to 2nd gear
2. 3.21 acceleration seems better in the 35-55 MPH road speeds

As an aside, on wet or snow covered road services the 3.92 can produce excessive wheel spin.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 17 July 2018. Now at: 029321 miles.
 

VernDiesel

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Well I was asking if you knew the same info with respects to the 2020 ED transmission. Its fine that you don't.

As to the engine. My actual long term ownership experience is that most of its problems are emissions system driven and can be dealt with. And that the engine & transmission themselves while not robust or designed for commercial towing as I use it are still an excellent & fuel efficient platform for towing 1/2 ton appropriate loads. It just tows so efficiently and with general ease in comparison to a naturally aspirated gas truck. While you have to get the exhaust brake from an aftermarket tune with the trucks having the for towing 10.5 quart oil capacity and 9,200 pound tow rating I'd say they were designed with towing in mind. Additionally the availability of factory bolted to frame receiver, trailer lighting & brake plug in connectors, tow mirrors, trailer brake controllers, and tow/haul setting for the transmission I believe Ram did in fact have towing in mind. As to the GM money saving aluminum block 3.0 diesel. Time will tell how it holds up when used for towing and gets the boost turned up or overheated from even just a simple blown radiator hose that all truck are susceptible to over time. Aluminum blocks are fine until something happens outside normal parameters. Then owners often learn that the stronger and still very light compared to nodular iron CGI blocks that VM Motori and now Cummins use can be money well spent. As to the exhaust brake that comes in the GM 3.0 diesel unfortunately Fast Lane Trucks found it to be nearly worthless as weakly as GM sent it out the door so it will still need aftermarket tuning to make it more than a novelty.

Below is one of my reply posts from another forum on this subject.

Congrats on the new rig and welcome. Most of us love this engine and truck some love to hate it. There are problems with the 2nd gen ED engines that we have in our 4th gen trucks. Mostly surrounding the emissions systems. First EGR; putting soot & tiny asphaltine hard particles into your motor ending up in your oil contributing to bearing failures IE catastrophic motor failures and generally clogging things up which lead to the coolant and melting intake fires you referenced. Later EGR cooler leaks that run you low on coolant which leads to havoc with the water pump radiator and even head gaskets. All compliments of EPA driven regulations and the short time frames they gave the Mfgrs to design test and bring it to market. Most uninformed consumers just blame the Mfgrs. All diesel pick ups have struggled with this some more than others but it seems newer designs will fare better in contending with these regulations. To make matters worse 14 thru 16s get an AEM tune forced upon them. But the $3k check that comes from it can more than pay to fix the problems that generally come with it and make the truck better than original with tunes & EGR cooler deletes. Unfortunately most consumers are unaware and then their is always the contention of warranty.

Anyway between The FIRST to offer 14-18 RAM 1500/JEEP Grand Cherokee EcoDiesel ECU & Transmission Tune via OBD2 flash! or Dodge Cummins & Eco Diesel - Eco Diesel Dodge Ram 1500 3.0L 2014-2019 or other companies like them.

And EGR Delete Kit (Stage 2 kit) or Eco Diesel Solution most of the problems are avoidable. Generally if you don't do the full delete IE only the basic tune(s) and leave your EGR cooler in place until post warranty (100k power train warranty that is) then the powertrain warranty doesn't get voided. Can that be garanteed.. nope. So the choice and potential consequences either way are on the truck owner. I guess your automotive philosophy and long term ownership plan come into play. Naturally both these things are only for your off road truck since apparently only politicians are allowed to break laws with impunity. Finally to drive/tow more safely with the more aggressive 60 HP high output tune settings you would also need Edge Product CTS2 Insight 84130 gauge monitor And Edge 98620 EAS Expandable EGT Probe Kit Hope this helps you hit the ground running.

BTW I tuned my first engine early (EGR gets turned off) and it went 371k miles towing travel trailers before low coolant stemming from an EGR cooler leak caused it to push a head gasket.(IMO) I chose to replace it instead of re-built it and that motor is going strong at 272k transporting travel trailers and boats from the Mfgrs to their dealerships. So in my experience these motors can hold up well even when worked hard when they are not being killed by the emissions systems well and get proper maintenance.
 
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Willwork4truck

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That might have been me. I'm a notorious do-not-carer when it comes to truck fuel economy. I accept that it sucks relative to virtually all other vehicles and just enjoy the ride, or pick one of my other vehicles that is more appropriate for the job. Like today, I needed to let someone in that locked themselves out of a property, and I wanted to stop and feed the ducks. Neither of which required the truck, which just came back from a 1500 mile rounder to and from Florida. :)
Best ever reply to mpg concerns...
 

Willwork4truck

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It shouldn't matter why someone is or isn't concerned about mpg... My employer pays for my fuel, but it still concerns me that I get the lowest range mpg of any other I see reported.

Saying "I don't care so no one else should" is goofy imo.
I'm not advocating complete ignorance, just not fretting over it too much. The respondant obviously enjoys driving his truck and so didn't hold fuel economy as #1, while there are folks who seek to squeeze out every last mile.
Having just sold a 2011 Prius a few months ago, I know all about looking for gas mileage. I also know that for me, the gas saved just wasn't worth the ride, the flimsy feel, the accident dangers and the lousy stereo.
If someone is getting really bad mileage then by all means get it checked out at the dealer. Too many times, "poor fuel mileage" is never fixable at the dealer due to the variables.
 

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