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

[neeginan]

Nice. I should do that for mine, but I'm lazy af. What school did you get that fancy degree from?

B. Sc. E. E. from a well known university.

I dont like to share too many details. I work in technology, know how tech companies abuse our information, and know that our digital footprints and fingerprints are already far too extensive!

 

[EEnginerd]

B. Sc. E. E. from a well known university.

I dont like to share too many details. I work in technology, know how tech companies abuse our information, and know that our digital footprints and fingerprints are already far too extensive!

Lol. Ditto (of course). And completely understand. Not that your truck details aren't basically a fingerprint or anything...

 

[neeginan]

Lol. Ditto (of course). And completely understand. Not that your truck details aren't basically a fingerprint or anything...

Agreed. Like I said, our digital footprints and fingerprints are already far too extensive!

 

[bkp]

Thanks for the time and effort to explain this. I'm sure this will help some people decide on what to spend their hard earned money on.

 

[Yogi217]

What kind of scenario, assuming you are operating within limits of load and towing, would 3.92 be able to do something 3.21 wouldn't?

I know 3.92 can get you "started" more easily, but once you are going, it's somewhat of a wash (in general terms and without getting into the weeds).

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[Scram1500]

What kind of scenario, assuming you are operating within limits of load and towing, would 3.92 be able to do something 3.21 wouldn't?

Crawl at a snails pace in 4Lo

Also turn larger diameter tires without affecting performance as much. Just going up one aspect ratio from the OEM 32" to a 33" tire changes the 3.92 to a 3.79. The 3.21 becomes 3.11

For a slightly more extreme example, 35" tires with 3.21 has an effective ratio of 2.95

 

[Yogi217]

Crawl at a snails pace in 4Lo

Also turn larger diameter tires without affecting performance as much. Just going up one aspect ratio from the OEM 32" to a 33" tire changes the 3.92 to a 3.79. The 3.21 becomes 3.11

For a slightly more extreme example, 35" tires with 3.21 has an effective ratio of 2.95

Thank you.

I wonder about going to a 33. 4th gen rams had 33 as stock and also had 3.21s. Why would it be different?

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[Scram1500]

Thank you.

I wonder about going to a 33. 4th gen rams had 33 as stock and also had 3.21s. Why would it be different?

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Fuel consumption and chasing EPA regulations. 33" are heavier than 32" therefore less fuel efficient

 

[Buz]

People always say it’s a wash after 2nd gear but I always compare the two ratios to the front sprocket on a 10 speed bike. The 3.92 is the smaller front sprocket and the 3.21 is the larger one.
Even if you match the ratios to get identical RPM’s (or pedal speed on the bike) it always felt easier to accelerate when on the smaller front sprocket. Maybe it’s just in my head.

 

[Yogi217]

Fuel consumption and chasing EPA regulations. 33" are heavier than 32" therefore less fuel efficient

I'm starting to think it's minimal at 33 for the user, but when combined across all the vehicles on the road, it looks better for the epa or whatever. The 4 gens had 33s and 3.21, the gmc and chevy trucks have some options that pair 33s and 3.23 rears.

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[Yogi217]

People always say it’s a wash after 2nd gear but I always compare the two ratios to the front sprocket on a 10 speed bike. The 3.92 is the smaller front sprocket and the 3.21 is the larger one.
Even if you match the ratios to get identical RPM’s (or pedal speed on the bike) it always felt easier to accelerate when on the smaller front sprocket. Maybe it’s just in my head.

I wonder if that's where (after 1st gear) the transmission mitigates the difference and levels out the difference for the engine?

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[Mike.3]

Great discussion. Follow up question. If the engine is the 6.0 liter diesel, and you are towing a 4500lb trailer, is the 3.92 necessary? Yes, it does increase the towing capacity, but the 3.21 capacity is well above planned towing. Thanks for your replies and input.

 

[Mike.3]

Sorry, miss typed! 3.0 liter V6 eco diesel.

 

[Morenar]

Other than the window sticker and build sheet. Is there a way to look under the truck and know for sure which ratio you have? Maybe some stamping. I have heard on other brand forums where people thought they got the right axle ratio and ended up not being what was on the sticker.

 

[Timeless]

Other than the window sticker and build sheet. Is there a way to look under the truck and know for sure which ratio you have? Maybe some stamping. I have heard on other brand forums where people thought they got the right axle ratio and ended up not being what was on the sticker.

Another way to know I would suspect is to compare your RPMs at a certain speed. Like post your RPMs at 70 MPH and I am sure people will let you know very quickly.

 

[Scram1500]

3.92 at approximately 70mph is 1900 rpm, the 3.21 at that same speed is around 1600 rpm

 

[Yogi217]

How much more torque does the 3.92 put down when starting from a stop vs 3.21?

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[Timeless]

How much more torque does the 3.92 put down when starting from a stop vs 3.21?

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Can we assume the 3.92 is around 200 rpm ahead of the 3.21 from a start to whatever you reach? If so, it should be fairly easy to figure out with a dyno graph....

 

[Yogi217]

Can we assume the 3.92 is around 200 rpm ahead of the 3.21 from a start to whatever you reach? If so, it should be fairly easy to figure out with a dyno graph....

where can I or we find that graph? I don't know jack about this stuff (obviosuly)...

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[Timeless]

where can I or we find that graph? I don't know jack about this stuff (obviosuly)...

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I suspect someone on here has dynoed their truck and posted it...maybe in the performance/engine mod section?