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

[IvoryHemi]

My overall mpg average (5.7 non-eTorque, 4x4, 3.92) for regular driving is 16.83 mpg while towing my travel trailer is 8.44 mpg

If I had a 4x2 my averages would be 1-2 mpg higher. I maintain that 4x4 vs 4x2 will make a bigger mpg difference than 3.21 vs 3.92

 

[BowDown]

My overall mpg average (5.7 non-eTorque, 4x4, 3.92) for regular driving is 16.83 mpg while towing my travel trailer is 8.44 mpg

If I had a 4x2 my averages would be 1-2 mpg higher. I maintain that 4x4 vs 4x2 will make a bigger mpg difference than 3.21 vs 3.92

Yet if I choose manual gear mode and lock it to 7th on the highway at 70mph, I lose about 4-5 mpg. 23 mpg down to 17-18 mpg

 

[mmcbeat]

I recently bought a Ram with the Hemi and 3.92 rear end and am happy with it. I’m going to make a comment and ask a question that will show my age. At one time a 3.92 rear end was considered very low for an everyday driver. But that was when high gear in the transmission had a ratio of 1:1. Now in the eight speed, 6th gear is 1:1 and 7th and 8th gears are overdrive. Does a Ram with 3.21 rear end cruise easily in 8th gear?

Asking Because I had a Jeep JLU with 3.6 engine, 8 speed auto, and 3.45 rear end. The Jeep spent a lot of time in 6th gear, occasionally 7th, rarely 8th.

 

[silver billet]

My overall mpg average (5.7 non-eTorque, 4x4, 3.92) for regular driving is 16.83 mpg while towing my travel trailer is 8.44 mpg

If I had a 4x2 my averages would be 1-2 mpg higher. I maintain that 4x4 vs 4x2 will make a bigger mpg difference than 3.21 vs 3.92

I'm going to disagree there heavily; on the freeway you are going to be running at 400 to 500 RPMs higher than me. You just can't come back from that kind of hit by dropping 100 pounds of weight or whatever it is.

 

[IvoryHemi]

Yet if I choose manual gear mode and lock it to 7th on the highway at 70mph, I lose about 4-5 mpg. 23 mpg down to 17-18 mpg

Over the course of how many miles?

 

[IvoryHemi]

I'm going to disagree there heavily; on the freeway you are going to be running at 400 to 500 RPMs higher than me. You just can't come back from that kind of hit by dropping 100 pounds of weight or whatever it is.

It’s only a 350 rpm difference at 70 mph. Won’t hit a 400 rpm gap until 80 mph and 500 rpm until 100 mph

Here is rpm using the 32.1” tire (285/45/22) size on 3.21 vs 3.92

60 mph: 1649 - 1350 = 299 rpm
65 mph: 1786 - 1463 = 323 rpm
70 mph: 1924 - 1575 = 349 rpm
75 mph: 2061 - 1688 = 373 rpm
80 mph: 2199 - 1800 = 399 rpm
100 mph: 2749 - 2251 = 498 rpm

Also, I believe 4WD adds ~250 lbs. (loss of 50 lbs payload because GVWR only increases by 200 lbs)

2 mpg may be optimistic, but 2WD easily adds 1+ mpg on the highway

 

[tidefan1967]

I recently bought a Ram with the Hemi and 3.92 rear end and am happy with it. I’m going to make a comment and ask a question that will show my age. At one time a 3.92 rear end was considered very low for an everyday driver. But that was when high gear in the transmission had a ratio of 1:1. Now in the eight speed, 6th gear is 1:1 and 7th and 8th gears are overdrive. Does a Ram with 3.21 rear end cruise easily in 8th gear?

Asking Because I had a Jeep JLU with 3.6 engine, 8 speed auto, and 3.45 rear end. The Jeep spent a lot of time in 6th gear, occasionally 7th, rarely 8th.

A Ram with 3.21s will easily stay in 8th gear on fairly flat ground it’s only when you get in to hilly country that it wants to downshift but it’s not too bad IMHO. My first ram had 3.21s, my previous and current trucks have the 3.92s and I prefer those. Honestly I don’t know how anybody would think that these trucks are slow they are way faster than any GM truck with a 5.3 I can tell you that and not far behind the current 5.0 in the Fords. The 3.5 Eco-boost is another story.

 

[brian42]

Does a Ram with 3.21 rear end cruise easily in 8th gear?

Yes it does. At 70 mph I'm running around 1600-1650 RPM and in 8th. I can even hold MDS active (ECO light) with a slight headwind or mild incline.

I go in and out of ECO rather than kick down out of 8th.

 

[BowDown]

Over the course of how many miles?

Over 7-10 miles on a drive from Dallas to Shreveport.
There's roughly a 19% difference between the 3.21 and the 3.92, there's also a pretty obvious 500 rpm difference between the two, roughly 20%.

What I observed was a drop from 23-24 mpg to 17-18 mpg when holding 7th. Using a % of change calculator, that's 21% loss.

 

[BowDown]

It’s only a 350 rpm difference at 70 mph. Won’t hit a 400 rpm gap until 80 mph and 500 rpm until 100 mph

Here is rpm using the 32.1” tire (285/45/22) size on 3.21 vs 3.92

60 mph: 1649 - 1350 = 299 rpm
65 mph: 1786 - 1463 = 323 rpm
70 mph: 1924 - 1575 = 349 rpm
75 mph: 2061 - 1688 = 373 rpm
80 mph: 2199 - 1800 = 399 rpm
100 mph: 2749 - 2251 = 498 rpm

Also, I believe 4WD adds ~250 lbs. (loss of 50 lbs payload because GVWR only increases by 200 lbs)

2 mpg may be optimistic, but 2WD easily adds 1+ mpg on the highway

I'm at about 1400-1500 rpm at 70. What's your rpm at 70?
Gear charts are OK for ballparking but don't take the additional parasitic loss lower gears cause. It's common in the corvette world to see dyno losses of 10-12 rwhp when going from 3.42 to a 3.90 or 4.10 which means the gear is costing you more power over the higher gear set

 

[BowDown]

I recently bought a Ram with the Hemi and 3.92 rear end and am happy with it. I’m going to make a comment and ask a question that will show my age. At one time a 3.92 rear end was considered very low for an everyday driver. But that was when high gear in the transmission had a ratio of 1:1. Now in the eight speed, 6th gear is 1:1 and 7th and 8th gears are overdrive. Does a Ram with 3.21 rear end cruise easily in 8th gear?

Asking Because I had a Jeep JLU with 3.6 engine, 8 speed auto, and 3.45 rear end. The Jeep spent a lot of time in 6th gear, occasionally 7th, rarely 8th.

Yes, cruises easily, hilly areas or traffic are the only times it comes out of 8th. You can also drive 80 mlh and still be under 2000 rpm.

 

[BowDown]

Yes it does. At 70 mph I'm running around 1600-1650 RPM and in 8th. I can even hold MDS active (ECO light) with a slight headwind or mild incline.

I go in and out of ECO rather than kick down out of 8th.

I'm about 100rpm less at the same speed, wonder if the big horn tires are a different size.

 

[IvoryHemi]

Over 7-10 miles on a drive from Dallas to Shreveport.
There's roughly a 19% difference between the 3.21 and the 3.92, there's also a pretty obvious 500 rpm difference between the two, roughly 20%.

What I observed was a drop from 23-24 mpg to 17-18 mpg when holding 7th. Using a % of change calculator, that's 21% loss.
View attachment 96541

That’s an extremely small data point.

An extra 350 rpm does not make the engine 22% less efficient at these speeds

 

[IvoryHemi]

I'm at about 1400-1500 rpm at 70. What's your rpm at 70?
Gear charts are OK for ballparking but don't take the additional parasitic loss lower gears cause. It's common in the corvette world to see dyno losses of 10-12 rwhp when going from 3.42 to a 3.90 or 4.10 which means the gear is costing you more power over the higher gear set

Yes dyno’s read lower but that’s not due to the lower gears having a higher inefficiency, it’s in how the dyno computer calculates the power. That’s a whole different discussion

 

[brian42]

I'm about 100rpm less at the same speed, wonder if the big horn tires are a different size.

I have OEM sized tires on my stock 18" rims but they are 41K miles old so the circumference is a little less than when they were new, which could account for the RPM difference.

 

[IvoryHemi]

OEM size tires are all basically the same height. There won’t be any noticeable RPM difference between any of them.

275/65/18 = 32.08”
275/55/20 = 31.90”
285/45/22 = 32.10”

 

[BowDown]

That’s an extremely small data point.

An extra 350 rpm does not make the engine 22% less efficient at these speeds

Yet the mpg dropped.
I'm not willing to burn 200 mpg of fuel just to prove the obvious difference between the 2 gearsets, if you're happy with 3.92's great. I wanted better fuel economy, I've been down the 3.92 road and I now average 420-430 miles per tank driving 80/20 city highway.
I've yet to see a 3.92 geared truck do that, 2 or 4 wheel drive. What's your rpm@ 70MPH ?

 

[BowDown]

Yes dyno’s read lower but that’s not due to the lower gears having a higher inefficiency, it’s in how the dyno computer calculates the power. That’s a whole different discussion

Thats dyno manipulation
The loss is caused by an increase in the parasitic loss, ie, now more rwhp is going up as heat instead of power, because of the teeth spacing/angle which very slightly changes the coefficient of friction. that's all. the car is still making the exact same flywheel hp, and there's nothing going on with any 'correction factors'.

 

[IvoryHemi]

Yet the mpg dropped.
I'm not willing to burn 200 mpg of fuel just to prove the obvious difference between the 2 gearsets, if you're happy with 3.92's great. I wanted better fuel economy, I've been down the 3.92 road and I now average 420-430 miles per tank driving 80/20 city highway.
I've yet to see a 3.92 geared truck do that, 2 or 4 wheel drive. What's your rpm@ 70MPH ?

Sure but mpg has fluctuations, it’s not a straight 6 mpg drop

400+ miles with mixed driving is achievable with 3.92, RPM’s are 22% higher, so if your at 1400 I’m at 1700

 

[IvoryHemi]

Thats dyno manipulation
The loss is caused by an increase in the parasitic loss, ie, now more rwhp is going up as heat instead of power, because of the teeth spacing/angle which very slightly changes the coefficient of friction. that's all. the car is still making the exact same flywheel hp, and there's nothing going on with any 'correction factors'.

If there was significantly more parasitic loss then yes it would, but there isn’t. That was debunked years (10+) ago on Corvetteforum.

It’s been a long time since I’ve read it but basically it was found that the difference is in relation to how the dyno calculates the time it takes the engine to rev from A to B. And within that calculation it ends up with an estimate that will show lower power.