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

[JF19Longhorn]

Manual recommends 89oct.

87oct can be run, but it might ping, again according to the manual.

 

[DTN]

The dealership put in regular and told me to use regular but I did see the recommendation. I just got my truck so I'm still burning the dealership gas. No problems so far but I will see how it goes. My 2003 would run rough if I put 87 in it so I had to use 89.

 

[brian42]

The dealership will put in whatever is closest and cheapest regardless of the owner’s manual recommendations.

 

[DTN]

The manual states you can use 87 to 89 octane.

 

[JF19Longhorn]

That must be new. Mine has the disclaimer for running 87 that it could be used but light pinging might occur and it was normal. ..or something along those lines

 

[DTN]

The manual does have a general statement on fuel requirements - gasoline engines on pinging and using lower than recommended octane and poor quality gasoline. But in the 5.7L Engine specific section it states 87 to 89 octane is required and they recommend 89 octane for optimum performance and fuel economy.

 

[brian42]

The Owner's Manual also recommends Top Tier gasoline:

 

[alvinm]

When shopping for my two wheel drive Limited, I noticed that the majority of the dealer's inventory had 3.92 gears, so I figured that's what Ram recommended. So how much of a difference in mpg is it between the two?

 

[Buz]

When shopping for my two wheel drive Limited, I noticed that the majority of the dealer's inventory had 3.92 gears, so I figured that's what Ram recommended. So how much of a difference in mpg is it between the two?

Highly debatable, but of everything I've read I'd say 2-3mpg.

 

[brian42]

Not a lot. Driving style and road conditions make more of a difference.

I've read of 3.21 guys getting 13 mpg and 3.92 guys getting 17 mpg. I get 15 mpg overall (hand calculated).

I wanted 3.92s but the truck I found had 3.21s. After driving it I was more than happy with the 3.21 gears. If I had any plans for regular towing or getting bigger (or E-rated) tires then I would have stuck to my guns and gotten the 3.92 rear end.

 

[Fauji Rebel]

I hope this post will help to end the debate with facts and not opinions, and become THE post people refer to those who are having a hard time deciding.

You already know that 3.92 is better for towing, and 3.21 gets better fuel economy, so I will talk about what you might not know

Bottom line up front:

In layman's terms, to conceptualize the difference, imagine

1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92.

2) 3.21 has "an extra" overdrive gear.

3) 3.21 has 2 unique lower gears for towing.

4) 3.92 has 3 unique lower gears for towing.

5) Speed range that 3.21 is better at towing: 31-38 MPH, 48-57 MPH.

6) Speed range that 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH.


Explanation


1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92:

Here's the gear ratio for the 8 speed transmission:
1) 4.71:1 2) 3.14:1 3) 2.10:1 4) 1.67:1 5) 1.29:1 6) 1.00:1 7) 0.84:1 8) 0.67:1 Reverse) 3.30:1

Final drive ratios with 3.21

1st. 15.12, 2nd. 10.10, 3rd. 6.74, 4th. 5.36, 5th. 4.14, 6th. 3.21, 7th. 2.70, 8th. 2.15, R 10.6

Final drive ratios with 3.92

1st. 18.46, 2nd. 12.31, 3rd. 8.23, 4th. 6.55, 5th. 5.06, 6th. 3.92, 7th. 3.29, 8th. 2.62, R 12.94

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 --

2) 3.21 has "an extra" overdrive gear:

The 8th gear in 3.92 is the 7th gear in 3.21, thus effectively mean the 8th gear in the 3.21 is an extra gear to the 3.92.

Meaning, when you go test drive the 3.21 you will have to downshift to 7th to get the same acceleration at 3.92's 8th on freeways. That is why some people complain about how "sloppy" the 3.21 is, because the 3.21 has an extra overdrive gear for fuel economy. If you shift 3.21 in 7th gear, you will get the same acceleration as the 3.92 in 8th on the freeway. No, 3.21 isn't sloppy, you're just in a gear that 3.92 does not have.

3) 3.21 has 2 unique lower gears for towing:

As we know from 1), 5 gears have the same final drive ratio.
You "gain" an overdrive gear, but you "lose" one towing gear.
Here's the final drive ratio for the 2 towing gears.
1st. 15.12, 2nd. 10.10,

4) 3.92 has 3 unique lower gears for towing:

Same logic as the last
Final drive for 3 towing gears.
1st. 18.46, 2nd. 12.31, 3rd. 8.23.

5) Speed range where 3.21 is better at towing: 31-38 MPH, 48-57 MPH,
AND
6) Speed range where 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH:

Calculated towing shift point to be 6000 rpm, if I'm off the logic is the same but the speed will vary.

For towing,
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.

From the speed 31-38 MPH, 3.21 has higher final drive ratio over 3.92 (15.12 vs 12.31) until it has to shift to 2nd gear at 38MPH.

From the speed 39-47 MPH, 3.92 has higher final drive ratio over 3.21 (12.31 vs 10.10) until it has to shift to 3rd gear at 47 MPH.

From the speed 48-57 MPH, 3.21 has higher final drive ratio over 3.92 (10.10 vs 8.23) until it has to shift to 3rd gear at 57 MPH.

From the speed 58-70 MPH, 3.92 has higher final drive ratio over 3.21 (8.23 vs 6.74) until it has to shift to 4th gear at 70 MPH.

The key takeaway here is that towing heavier trailers uphill with 3.21 might never reach the desired speed within the 58-70 MPH range (typical highway towing speed) because 3.21 jumps from 10.10 to 6.74 without the 8.23 final drive ratio found in 3.92 that really help maintaining highway towing speed at max load.

Do you value the "extra" overdrive gear for fuel economy? or do you value the extra towing capability that you tell yourself you might one day need? That's up to you.

 

[Fauji Rebel]

Nicely explained

 

[Royalist_Ram]

The Owner's Manual also recommends Top Tier gasoline:

View attachment 83598

Me with my EcoDiesel that finally came in be like....

 

[c3k]

@Jack , Thanks for doing this.

By showing the various final gear ratio comparisons, this prompted me to mate my ecoDiesel to a 3.21 final drive. I'd just assumed a 3.92 was the way to go. But, given the ecoDiesel's torque, the actual ratios, and my planned use, I went 3.21.

If I regret it, I'll let you know.

 

[bwsRam19]

wish they offered 4:10 gears in the half ton, but oh well, 3.92 will have to do

 

[theblet]

Nice thread. I don’t do a lot of towing, but commute about 30 miles to work so I went with 3.21s. At least I have the 5.7 if needed.

 

[Craig777]

Great explanation, although it will not end the debate, as folks feel they need to defend their purchases to the death regardless of facts.
Thanks for putting the time and effort on this write-up. Should be a sticky.

Cheers,

Excellent write up, aren’t Engineers great (one here of course!

 

[jimchi]

I made a spreadsheet that calculates vehicle speed for a given RPM vs. gear. These calcs use the stock tire size on a 20" wheel, and add 3% to the wheel speed for loading (per tiresize.com). I used these charts to help in my decision for gearing. Since I do a lot of highway cruising, I was interested in differences between 1000 and 2500 RPM, and that's why you'll see more resolution there in the RPM column. Keep in mind that some of these results are unrealistic (e.g. going 21mph in 8th gear), but I left in for cuiriosity's sake. I removed any values above 130mph so the color scale would look better.

 

[bwsRam19]

So glad I went 3.92 after looking at the scale, thanks.

 

[bwsRam19]

Excellent write up, aren’t Engineers great (one here of course!

Actually in my line of work, most of them lack common sense, but anyway.