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

[HSKR R/T]

I think that there's a "want" for the 3.92 gear set to deliver superior performance and if it were being used with a 4 or 5spd trans, they'd be correct. Despite post 1 and the math, many are not getting what the 8 spd trsns actually accomplishes.

Even if the transmission shifts as you suggest and will keep the transmission gear being used different to maintain equal RPMs the information in the first post tells me that there are no real "matches" between the two. Close on some, sure, but some aren't what I would call a "match".

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

 

[SpeedyV]

I'm sorry but "we don't have the answers" is completely false. The math doesn't lie.

Read (for example) dean77's recent post again. I mean it's completely obvious the math is escaping 99% of you guys.

Man, I already agreed that the math is simple; please pay attention! Thanks for keeping an open mind to the possibility that shift mapping is programmable

 

[BowDown]

Even if the transmission shifts as you suggest and will keep the transmission gear being used different to maintain equal RPMs the information in the first post tells me that there are no real "matches" between the two. Close on some, sure, but some aren't what I would call a "match".

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

Uhh what?
The last 2 days of comments have been how the only real difference are gears 1,2 and 8 which you initially say no match then say they match.
If we apply your logic of the bigger lever rear gear easier go accelerate, the 3.21 has a "better" effective ratio from 3rd through 8th so if your argument was accurate, the 3.21 is better around town once out of 2nd gear too.

 

[silver billet]

Even if the transmission shifts as you suggest and will keep the transmission gear being used different to maintain equal RPMs the information in the first post tells me that there are no real "matches" between the two. Close on some, sure, but some aren't what I would call a "match".

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

Actually the numbers there in the first post are for the older 8 speed, apparently we in the 2019's have an updated transmisison. The numbers are slightly different, but the idea is the same.


The matches are in the various color shades. So the darkest blue shows how the 3.21 in 3rd gear, matches the 3.92 in 4th gear, with a difference of only 0.127.

So yes. With difference values of 0.127, 0.004 etc; they are absolutely a match for eachother.

 

[silver billet]

Uhh what?
The last 2 days of comments have been how the only real difference are gears 1,2 and 8 which you initially say no match then say they match.
If we apply your logic of the bigger lever rear gear easier go accelerate, the 3.21 has a "better" effective ratio from 3rd through 8th so if your argument was accurate, the 3.21 is better around town once out of 2nd gear too.

That's a great point as well. For every one of the matches except one, the 3.21 actually has the slightly higher gear ratio, which means higher RPM/power.

 

[Buz]

Isn't there something to be said for the fewer teeth on the 3.21 rear gear?
Wouldn't there always be more strain on that gear, and everything up the line [driveshaft/transmission/engine] if both gearsets are covering the same ground over the same time?

 

[tjsadler]

I love that an engineer took the time to show the exact differences with math and actual numbers and yet this thread is 18 pages long. I keep coming back (with popcorn in hand) because it's just SO entertaining to read how people believe their butt dyno over science. Interesting microcosm of the world of truthiness.

 

[Buz]

I love that an engineer took the time to show the exact differences with math and actual numbers and yet this thread is 18 pages long. I keep coming back (with popcorn in hand) because it's just SO entertaining to read how people believe their butt dyno over science. Interesting microcosm of the world of truthiness.

Agree this has been taken to an almost absurd level, but it's fun and I personally want to know why 3.92 is still such a popular option if the differences between the two gearsets is negligible?
Let look at the facts:
-MDS was implemented to save 1-2 MPG. I'm sure the corporate costs to implement MDS / Frame Shakers, etc tech into their vehicles was staggering.
-Retractable airdams to save .3 MPG
-Lowering suspension to save .5 MPG
-Alternators that hold-off on charging until the last second to save .1 to .2 MPG
-Low rolling resistance tires
-Grill shudders
-And the list goes on and on to save minuscule MPG's

YET:
FCA offers the 3.92 in plentiful numbers which KILLS the MPG's 1-3MPG (HUGE) that they've struggled so hard to achieve? Why?
Why not just offer the 3.21 with an emphasis on educating the buying public why high gear ratios (low gear) are a 'thing of the past' with modern 8+ speed transmissions. Nobody should be towing more than 8.000lbs with a half ton anyway. Why is RAM killing their reputation with unnecessary bad MPG numbers.
Is the 3.92 just politics?

 

[JF19Longhorn]

Wow...

 

[Rossum]

-MDS was implemented to save 1-2 MPG. I'm sure the corporate costs to implement MDS / Frame Shakers, etc tech into their vehicles was staggering.
-Retractable airdams to save .3 MPG
-Lowering suspension to save .5 MPG
-Alternators that hold-off on charging until the last second to save .1 to .2 MPG
-Low rolling resistance tires
-Grill shudders
-And the list goes on and on to save minuscule MPG's

All this stuff adds up and helps them get better ratings from the EPA. This is critical; they have some "fleet average" numbers they have to meet, otherwise there are penalties.

FCA offers the 3.92 in plentiful numbers which KILLS the MPG's 1-3MPG (HUGE) that they've struggled so hard to achieve? Why?

Because some people want it. There's no question that it puts more torque into the axles while the truck is in first gear, and since it's optional and costs money, it doesn't affect the EPA mileage ratings. Same thing with other options that add weight; people want them, but the EPA ignores their effect on mileage.

 

[tjsadler]

Honestly, I think a lot of the options offered are mostly marketing. If you look at the Ford site the F250 with a Powerstroke stock axle ratio is 3.31 if I'm not mistaken. Kind of like listing 12,500lb towing capacity on a half ton. Can it? Yes. Should it? No. For that matter the 35k lb towing capacity of the HD trucks. Sure they might have a few that tow that heavy with a 3500 but I'd guess that most people will never tow that much without having a larger truck. If they didn't at least offer the 3.92 people would be roasting them for not beating the specs of their competitors. Most of the 250/2500 trucks I see are lifted and customized in ridiculous (to me) ways that have more to do with the owner's genitalia size (or lack thereof) than any actual needed functionality. If you have and like the 3.92, great! Just don't act like it gets some magical amount of additional capacity for 99% of the use cases of a half ton truck. If you like to lift your truck 8" and run 38" tires from your suburban home to your office, more power to you! Enjoy! Stop arguing with almighty math, though.

 

[silver billet]

Agree this has been taken to an almost absurd level, but it's fun and I personally want to know why 3.92 is still such a popular option if the differences between the two gearsets is negligible?
Let look at the facts:
-MDS was implemented to save 1-2 MPG. I'm sure the corporate costs to implement MDS / Frame Shakers, etc tech into their vehicles was staggering.
-Retractable airdams to save .3 MPG
-Lowering suspension to save .5 MPG
-Alternators that hold-off on charging until the last second to save .1 to .2 MPG
-Low rolling resistance tires
-Grill shudders
-And the list goes on and on to save minuscule MPG's

YET:
FCA offers the 3.92 in plentiful numbers which KILLS the MPG's 1-3MPG (HUGE) that they've struggled so hard to achieve? Why?
Why not just offer the 3.21 with an emphasis on educating the buying public why high gear ratios (low gear) are a 'thing of the past' with modern 8+ speed transmissions. Nobody should be towing more than 8.000lbs with a half ton anyway. Why is RAM killing their reputation with unnecessary bad MPG numbers.
Is the 3.92 just politics?

Let me just say (since I've been arguing on this thread a lot) that I also find it far more "fun" than "upsetting". So there is no intended malice on my part and I hope no one has been offended.

The 3.92 serves a purpose, that's not the issue. For example, it works well in the Rebel where the intent is to be used offroad for rock climbing. You want every last bit of low end torque you can get.

It also works well when pulling very heavy loads, from a stop. There are 2 things to keep in mind there though; A) It's from a start only, not once you're moving in 3rd and beyond. B) The rest of the truck has to be able to handle towing the load, and the weakness of the 1500 happens to be payload far more often than it is power. If you want to tow heavy with a 1500, you're basically stuck in tradesman/big horn, maybe a few exceptions here and there in higher trims.

Finally there are those who insist on getting to the next stoplight before you, and hey, every bit of low end torque helps there too.

So no, the 3.92 is not just politics. But there does seem to be an awful lot of people buying it just because, without realizing its less effective than they think it is. For the right buyer, it's an excellent option.

 

[BowDown]

Isn't there something to be said for the fewer teeth on the 3.21 rear gear?
Wouldn't there always be more strain on that gear, and everything up the line [driveshaft/transmission/engine] if both gearsets are covering the same ground over the same time?

That's backwards, it's the numerically higher gears that are weaker.
The pinion gear is larger which makes the ring gear far thinner and weaker. That is typically a problem on small differentials though. I believe the ram is 9.25 or something

 

[flyfingers]

I absolutely wish to thank the OP for their time / effort in explaining all this. Like others have said, this is totally absurd at this point but still entertaining none the less. With that being said I must admit I thoroughly enjoy the banter and being as I have not purchased a truck yet this thread has swayed my decision in gear selection back and forth to the point that, when the time comes, I have no idea which gear sets to order. I do think I would eventually go up a size or two in tires simply for added ground clearance as well as ride quality which leans me toward 3.92's. If I can effectively achieve somewhere around 3.55 - 3.73 via tire selection well then I believe I'd have the best of both worlds - I mean as long as we're splitting hairs I might as well take it to the next level.

 

[Rossum]

Finally there are those who insist on getting to the next stoplight before you, and hey, every bit of low end torque helps there too.

And then there are people like me who insist on getting the highest mileage (and thus range on a tank) possible at 80 mph on the highway, and hey, every bit of engine RPM you reduce helps there too.

 

[Scram1500]

So... how about those catch cans?

 

[bwsRam19]

So 3.92 it is then, thanks everyone

 

[Calsun]

Gas engines need to operate at much higher rpm's to get into their peak power band and so taller gears work against the engine. When rock crawling or towing a heavy trailer the engine provides more usable torque and horsepower with lower gears. The idea that 3.21 gears are going to make a significant difference in fuel economy is ludicrous to the extreme.

The engine at low vehicle speeds is working to move the mass of the truck and get its wheels spinning. At freeway speeds 90% of the horsepower is used to overcome air drag and trucks have the aerodynamics of a brick and generate a great deal more air drag than any passenger car made in the last 50 years. If someone wants to get more miles per gallon with their pickup truck then they need to drive slower as the air drag increases exponentially with speed so there is 4x as much air drag at 80 mph as there is at 40 mph.

If you never tow and never take your truck off the road then accept the factory default 3.21 gears and don't worry about it.

 

[BowDown]

Gas engines need to operate at much higher rpm's to get into their peak power band and so taller gears work against the engine. When rock crawling or towing a heavy trailer the engine provides more usable torque and horsepower with lower gears. The idea that 3.21 gears are going to make a significant difference in fuel economy is ludicrous to the extreme.

The engine at low vehicle speeds is working to move the mass of the truck and get its wheels spinning. At freeway speeds 90% of the horsepower is used to overcome air drag and trucks have the aerodynamics of a brick and generate a great deal more air drag than any passenger car made in the last 50 years. If someone wants to get more miles per gallon with their pickup truck then they need to drive slower as the air drag increases exponentially with speed so there is 4x as much air drag at 80 mph as there is at 40 mph.

If you never tow and never take your truck off the road then accept the factory default 3.21 gears and don't worry about it.

Lol

 

[Rossum]

The idea that 3.21 gears are going to make a significant difference in fuel economy is ludicrous to the extreme.

Take two otherwise identical trucks, one with 3.21 and the other with 3.92 gears running side-by-side at a steady speed on the highway. Both engines will be producing exactly the same horsepower, but the engine in the one with the 3.21 gears will be turning ~20% more slowly. Lower revs mean lower friction losses in the engine. It also means the slower-turning engine will be running with higher manifold pressure, which means lower pumping losses. Is the difference "significant"? Hard to say, but it definitely exists, and I bet it would be measurable if we actually performed this experiment as described.