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

[Rick3478]

You guys really believe that engine rpm is the key to milage? My 4000 lb car has a 6.4l, bigger cam, runs 500 rpm higher than my little 5.7l truck. The car gets 4 mpg better. Why? It does not work as hard at a higher rpm to move. Kind of like the truck with proper 3.92 gears as opposed to the truck with underpowered 3.21 gears lugging along. It's not all about rpm. If it were the 6.4 spinning faster would burn more fuel.

Not the whole key, but a significant factor. Your car probably has much better aerodynamics, so apples and oranges.

But comparing different drive ratios (which includes tire size) for otherwise equal vehicles, I would expect lower RPM to be more efficient. I don't have a fuel consumption map specific to our 5.7, if anyone does I would love to have a copy, but a typical ICE efficiency "sweet spot" might be around 1200 RPM at 3/4 throttle. Lighter throttle causes more pumping loss, higher RPM more friction. My $.02.

 

[Mountain Whiskey]

Yes a car has better aerodynamics. It is a little lighter also but still rpm to rpm. I still contest that truck to truck, rpm is not making some big difference in fuel milage.
Another car case is my Lincoln. Way underpowered with a 4.6l motor. If I consistently turn off the overdrive when driving under 70 it gets about 1.5 mpg better than just letting it shift into overdrive when it wishes. It typically does this around 40 unless you are accelerating hard. You can hear the engine bog down. Lower rpm and it has to work harder.
In a Jeep (less aerodynamic than a cow!) I put bigger tires on once without regearing and the milage dropped like a rock. It had to work to hard to move itself.

All much like a 3.21 geared truck working harder to move the mass.

I think you see better milage in a 3.21 truck because the owners are more apt to keep it stock (you know, for picking up kids at soccer practice, groceries and makeup) Lifts and bigger tires kill milage way, way more than a couple hundred rpm. My truck seems to have suffered a 1mpg hit with tires that are 4 pounds heavier. Maybe the more aggressive tread affects it some, I am not sure. I need to set the speedo by rotation per mile rather than advertised size, to make sure it is exact as I can. I have not made that move yet.

I attribute my lower milage to large heavy tires, lift kit and a heavy foot way more than engine rpm.

 

[2021EcoDiesel]

You guys really believe that engine rpm is the key to milage? My 4000 lb car has a 6.4l, bigger cam, runs 500 rpm higher than my little 5.7l truck. The car gets 4 mpg better. Why? It does not work as hard at a higher rpm to move. Kind of like the truck with proper 3.92 gears as opposed to the truck with underpowered 3.21 gears lugging along. It's not all about rpm. If it were the 6.4 spinning faster would burn more fuel.

Load is key! Especially for diesels

 

[BowDown]

You guys really believe that engine rpm is the key to milage? My 4000 lb car has a 6.4l, bigger cam, runs 500 rpm higher than my little 5.7l truck. The car gets 4 mpg better. Why? It does not work as hard at a higher rpm to move. Kind of like the truck with proper 3.92 gears as opposed to the truck with underpowered 3.21 gears lugging along. It's not all about rpm. If it were the 6.4 spinning faster would burn more fuel.

You're comparing a car to a truck, another car setup the same but with a numerically lower gear will have better mpg. No different than the trucks here now.
At 60-65 mph, my 3.21 gear'd truck is indeed lugging and fuel mileage is around 17-18 mph but thats about 1300-1400 rpm, too slow for an engine that seems most efficient around 1600-1900 rpm.
At 70-75 mph, my mpg increases significantly to 23 mpg @70 and about 21 @ 74-75 mph. RPM to RPM the trucks should be the same save for rotational losses but the mpg will be different.

A 3.92 at 70 mph isn't getting the mileage a 3.21 is at 70 mph, that's fact.
Reading complaints here, most 3.92 trucks are doing good to get 17-18 mpg on the highway; I get that in the city.



80% city driving, I haven't driven a long distance highway trip in a year

 

[Darksteel165]

Yes a car has better aerodynamics. It is a little lighter also but still rpm to rpm. I still contest that truck to truck, rpm is not making some big difference in fuel milage.
ad to work to hard to move itself.

All much like a 3.21 geared truck working harder to move the mass

3.21 gears in a Ram will provide better mpg at speeds over 70mph, that is a fact.
With 3.92s we don't have the same gears, and thus are using more fuel because we go into gas guzzzler mode at a lower top speed then 3.21. 3.21s can go a bit further before they tap out in 8th gear and go into gas guzzzler mode.
At speeds at or lower then around 60 I would say it's a tosssup and I would be inclined to agree 3.92s provide better mpg but not at highway speeds (65+) where most people make the mpg gains on a tank of gas I would say which is why we see "3.21s get better mpg" which is a loaded statement.

 

[BowDown]

Not the whole key, but a significant factor. Your car probably has much better aerodynamics, so apples and oranges.

But comparing different drive ratios (which includes tire size) for otherwise equal vehicles, I would expect lower RPM to be more efficient. I don't have a fuel consumption map specific to our 5.7, if anyone does I would love to have a copy, but a typical ICE efficiency "sweet spot" might be around 1200 RPM at 3/4 throttle. Lighter throttle causes more pumping loss, higher RPM more friction. My $.02.

Which is exactly the theory behind mds, reduction in pumping losses.

What Is Hemi MDS?

We explain how Chrysler Hemi MDS cylinder deactivation works, what goes wrong, and how to fix it when it breaks.

www.motortrend.com

 

[BowDown]

3.21 gears in a Ram will provide better mpg at speeds over 70mph, that is a fact.
With 3.92s we don't have the same gears, and thus are using more fuel because we go into gas guzzzler mode at a lower top speed then 3.21. 3.21s can go a bit further before they tap out in 8th gear and go into gas guzzzler mode.
At speeds at or lower then around 60 I would say it's a tosssup and I would be inclined to agree 3.92s provide better mpg but not at highway speeds (65+) where most people make the mpg gains on a tank of gas I would say which is why we see "3.21s get better mpg" which is a loaded statement.

I partially agree, around town the 3.92 truck "should" get better fuel mileage for the simple fact that the 3.92 should be a bigger better lever for moving weight.
The only problem with that thought is that damn near all the 3.92 people that care about mpg complain about the 3.92s economy on both city and highway driving.

It would seem to me that once acceleration stops and you're driving a steady speed, whether that be 500 yards or 2- 3 miles, the truck turning the lower rpm still seems to use the least fuel. At 45 mph, my truck is getting ~36 mpg and in 6th or 7th gear. Seems to me the trans equals out the gears in city driving, on the highway, it cant.

 

[Ramroo]

3.21 gets about 2 sometimes 3 mpg better than 3.92 truck from my experience with two stock trucks doing the same thing. I attribute it to mainly lower rpm at cruise speeds.

 

[Mountain Whiskey]

You're comparing a car to a truck, another car setup the same but with a numerically lower gear will have better mpg. No different than the trucks here now.
At 60-65 mph, my 3.21 gear'd truck is indeed lugging and fuel mileage is around 17-18 mph but thats about 1300-1400 rpm, too slow for an engine that seems most efficient around 1600-1900 rpm.
At 70-75 mph, my mpg increases significantly to 23 mpg @70 and about 21 @ 74-75 mph. RPM to RPM the trucks should be the same save for rotational losses but the mpg will be different.

A 3.92 at 70 mph isn't getting the mileage a 3.21 is at 70 mph, that's fact.
Reading complaints here, most 3.92 trucks are doing good to get 17-18 mpg on the highway; I get that in the city.

View attachment 181420

80% city driving, I haven't driven a long distance highway trip in a year

The Lincoln comparison is the same car with different gears by locking out overdrive during the majority of the speed driven at.

You are comparing a half truck. Most are driving 4x4.

 

[E.Hands]

Anyone that thinks that 3.21 gears are "bogging down" the Hemi at any point, needs to research exactly what volumetric efficiency and peak torque are.
And why they both occur at the same RPM.

 

[Ddubya]

I have had a 2023 eTorque 3:92 with 18 inch tires for about six months. It has much more quickness than my 2020 3:21 with 20 inch tires in all gears. Mileage is not as good as the 3:21 but still around 19 in town and up to 23 on the highway if I keep it around 65. I have towed a 6500 pound trailer with no problem with the 3:92 where the 3:21 was more sluggish with a 5000 pound trailer. The addition of LT tires and Timbren SES make the 3:92 a nice tow vehicle for smaller trailers

 

[theblet]

Mine bogged down one time, wouldn’t move an inch. But I forgot the trans was still in park.

 

[E.Hands]

Mine bogged down one time, wouldn’t move an inch. But I forgot the trans was still in park.

Were you then grabbing for a column shifter that wasn't there?
Been there, done that...

 

[RVTRKN]

Everyone is in left field now, and is overthinking the differences in the ratio's.

3:92 = moving heavier loads easier and engine longevity pulling heavy = less MPG.

3:21 = maximizing your MPG, as well as lowering your engine RPM = better MPG.

If regularly towing heavy, 3:92 is the best option, MPG is a secondary thought. If cruising and light loads with occasional towing, 3:21 is a better fit.

I just towed my Pontoon (Tritoon) about 40 miles and wished I had 3:92's, but thats only a small percentage of the life of my 1500.

 

[BowDown]

The Lincoln comparison is the same car with different gears by locking out overdrive during the majority of the speed driven at.

You are comparing a half truck. Most are driving 4x4.

Yeah and it sounds like you're talking about a 90's-2000's lincoln with a 4 speed, not a modern car with a modern 6, 7, 7 or 10 spd trans so not a valid comparison.
If you're driving a 4x4, stop crying about the mileage, you knew that when you bought it and your 4x4 is doing the same thing my 4x2 is 99% of the time

 

[Rick3478]

Which is exactly the theory behind mds, reduction in pumping losses.

What Is Hemi MDS?

We explain how Chrysler Hemi MDS cylinder deactivation works, what goes wrong, and how to fix it when it breaks.

www.motortrend.com

Yes. I didn't mention it since this is an axle ratio thread (which spun off the road into tire size, mileage, and other stuff). MDS adds another dimension, fewer cylinders working harder to make the same power.

 

[BowDown]

Yes. I didn't mention it since this is an axle ratio thread (which spun off the road into tire size, mileage, and other stuff). MDS adds another dimension, fewer cylinders working harder to make the same power.

This thread spun off the road 3 years ago; 4x4, 3.92 nor the mighty trx is getting us out of this ditch

 

[E.Hands]

This thread spun off the road 3 years ago; 4x4, 3.92 nor the mighty trx is getting us out of this ditch

I started to leave some good data here but realized that you and others had already provided the same info back in 2021

 

[Rick3478]

This thread spun off the road 3 years ago; 4x4, 3.92 nor the mighty trx is getting us out of this ditch

Need a 3500 dualie with 4.56's at least.

 

[BowDown]

Need a 3500 dualie with 4.56's at least.

Naw man, we need a D12 dozer for this