3.21 vs 3.92 real world

Bsi13 · Jul 15, 2022

I realize there's a lot of posts about this but none game me the answer i was looking for. I'm just wondering given that the bulk of my travel is done between 80Km/hr and 90Km/hr, (50-55 mph) as well as in town, highway trips aren't very frequent. Would the 3.92 suit me better? I have the 3.21 now and it always seems to be in and out of 8th, I'm just wondering if fuel economy would actually be any different in this scenario. Reason I'm asking is that what I'm looking for in a truck is hard to find with the 3.21 but there are a couple 3.92's. Thanks

Rlaf75 · Jul 16, 2022

Go to your dealer and drive one so you can get a first and feel for it. If you like the differences then you made your decision. I doubt you'll notice much difference in mpg between the two if you're doing any city driving

evolizzee · Aug 2, 2022

Yeah, the 3.21 8speed constantly bounced between 7 and 8 when cruising at speed while going up and down hills when I first got mine. I live in KY so there's a lot of hills here. It was very annoying. Luckily, tuning got rid of that issue for me. But yes, the 3.92 set up does not hunt for a gear like the 3.21 does while cruising up and down hills. Tuning makes the 3.92 even more enjoyable to drive at cruising speeds. I don't think you'll notice very much of a difference between fuel economy either.

69GWC · Aug 11, 2022

Never had a problem with my 3:21s truck ran great and would easily smoke the tires or pull my 7500lb trailer even with 35s.
That said I bought it with those gears because nobody had a truck I wanted with 3:92s which would have been better to have with my 35s.

Bigskyroadglide · Aug 13, 2022

My 3.92 truck gets better fuel economy on the highway than my 3.21 truck.

It town they are about the same

pacofortacos · Aug 13, 2022

Bsi13 said:
I realize there's a lot of posts about this but none game me the answer i was looking for. I'm just wondering given that the bulk of my travel is done between 80Km/hr and 90Km/hr, (50-55 mph) as well as in town, highway trips aren't very frequent. Would the 3.92 suit me better? I have the 3.21 now and it always seems to be in and out of 8th, I'm just wondering if fuel economy would actually be any different in this scenario. Reason I'm asking is that what I'm looking for in a truck is hard to find with the 3.21 but there are a couple 3.92's. Thanks

Under these conditions, the 3.92's will be a good choice. I have the 3.92's. MPG should be the same or maybe even slightly better - depending on how much you enjoy the 3.92's

Wirerat · Aug 14, 2022

My 2017 ram qc 3.21 275/60/20 50k miles vs My new to me 2020 cc 3.92. 275/55/20 9k miles. Both 4x4.

The 4th Gen had taller gears and taller tires and lighter cab configuration.

I'm not seeing worse gas mileage. It's the same. Kinda bizarre how simular it is looking over my gas app history.

26 miles round trip daily, so relatively short which could play a roll here. I use the cruse control at every opportunity.

For me, through my unscientific testing comparing pretty different trucks there was no mpg loss going to the 3.92.

mikeru · Aug 31, 2022

I've owned 5th gens with both 3.21 and 3.92 gearing. For my commute, I don't notice much difference in them. And since I don't own anything that needs to be towed, that's a non issue for me. If I were choosing between two comparably equipped trucks, I'd probably take the 3.92 because they seem to be a little more desirable when it comes time to sell. But I wouldn't let the gear sway me in one direction or another on trucks that were equipped differently. I'd choose the truck with the options I'm looking for, regardless of which gear it has. Of course, If were to place an order I'd select 3.92, for the same reason I already mentioned.

Rav3ncroft · Aug 31, 2022

Don't be afraid to get the 3.92's.

Bsi13 · Sep 1, 2022

Thanks everyone for the replies

mikepol2 · Sep 9, 2022

I think a lot has to do with how hilly your area is. I have the 3.92's and get about 15.5 mpg city / 21 hwy, and it's very hilly here. So I probably use less gas pedal going up hills than if I had 3.21's. Had a Silverado with 4.10's years ago after owning several with lower gear ratios and noticed no decrease in mpg.

pacofortacos · Sep 9, 2022

Bsi13 said:
I realize there's a lot of posts about this but none game me the answer i was looking for. I'm just wondering given that the bulk of my travel is done between 80Km/hr and 90Km/hr, (50-55 mph) as well as in town, highway trips aren't very frequent. Would the 3.92 suit me better? I have the 3.21 now and it always seems to be in and out of 8th, I'm just wondering if fuel economy would actually be any different in this scenario. Reason I'm asking is that what I'm looking for in a truck is hard to find with the 3.21 but there are a couple 3.92's. Thanks

Pretty sure I said that they would be perfect for your use as that is where the 3.92's really shine. It will mainly be in how nice it is to drive though, mpg is going to be the same as the 3.21 or just a bit better - depending on the driver.

ramffml · Sep 9, 2022

mikepol2 said:
I think a lot has to do with how hilly your area is. I have the 3.92's and get about 15.5 mpg city / 21 hwy, and it's very hilly here. So I probably use less gas pedal going up hills than if I had 3.21's. Had a Silverado with 4.10's years ago after owning several with lower gear ratios and noticed no decrease in mpg.

Unless you're going up the hill in first gear, there is no difference. The transmission gear is multiplied by the rear end gear, to get your "gear ratio". Which means you can change either the transmission gear (down/up shift) or the rear end (3.21 to 3.92) and still end up with the exact same gear ratio going up that hill.

mikepol2 · Sep 9, 2022

ramffml said:
Unless you're going up the hill in first gear, there is no difference. The transmission gear is multiplied by the rear end gear, to get your "gear ratio". Which means you can change either the transmission gear (down/up shift) or the rear end (3.21 to 3.92) and still end up with the exact same gear ratio going up that hill.

What I meant was, you may be less inclined to give it more gas to make it downshift.

ramffml · Sep 9, 2022

mikepol2 said:
What I meant was, you may be less inclined to give it more gas to make it downshift.

Why would that be the case when both trucks would be running at the same RPM at the same speed making the same HP/Torque when approaching that hill? There is literally no difference, some simple math to illustrate:

2 (transmission gear ratio) * 4 (rear gear ratio) = 8 (wheel gear ratio)
4 (transmission gear ratio) * 2 (rear gear ratio) = 8 (wheel gear ratio)

Doesn't matter which way you swap those, you end up with 8 at the end.

In real life it's actually something like:
1 (6th gear in tranny) * 3.21 (rear gear ratio) = 3.21 (wheel gear ratio)
0.82 (7th gear in tranny) * 3.92 (rear gear ratio) = 3.2144

Notice how both trucks end up with virtually identical wheel gear ratio when the 3.21 is in 6th vs the 3.92 in 7th.

Your rear gear ratio becomes irrelevant when you have tons of gears for the transmission to play with. If you had only 3 or 4 gears like trucks 20+ years ago, different story.

mikepol2 · Sep 9, 2022

ramffml said:
Why would that be the case when both trucks would be running at the same RPM at the same speed making the same HP/Torque when approaching that hill? There is literally no difference, some simple math to illustrate:

2 (transmission gear ratio) * 4 (rear gear ratio) = 8 (wheel gear ratio)
4 (transmission gear ratio) * 2 (rear gear ratio) = 8 (wheel gear ratio)

Doesn't matter which way you swap those, you end up with 8 at the end.

In real life it's actually something like:
1 (6th gear in tranny) * 3.21 (rear gear ratio) = 3.21 (wheel gear ratio)
0.82 (7th gear in tranny) * 3.92 (rear gear ratio) = 3.2144

Notice how both trucks end up with virtually identical wheel gear ratio when the 3.21 is in 6th vs the 3.92 in 7th.

Your rear gear ratio becomes irrelevant when you have tons of gears for the transmission to play with. If you had only 3 or 4 gears like trucks 20+ years ago, different story.

OK if you want to get all mathy…

The only reason we care about gear ratio is because it controls the force applied to the road by the tires.

At the same RPM and in the same transmission gear, a truck with 3.92’s applies more force to the road than a truck with 3.21’s. Therefore at the same amount of fuel via the gas pedal it’s easier to go up a hill with 3.92’s. To climb the same hill at the same speed with 3.21’s you need more gas.

ramffml · Sep 9, 2022

mikepol2 said:
At the same RPM and in the same transmission gear, a truck with 3.92’s applies more force to the road than a truck with 3.21’s.

Your mistake is in thinking both trucks are in the same gear always, at the same time. They're not. The transmission gear is chosen based on throttle/load, and MPH.

When your truck is in 7th, mine will be in 6th. Doing the math, that means both trucks are putting the same force to the wheels.

IDSandman · Sep 9, 2022

mikepol2 · Sep 9, 2022

ramffml said:
Your mistake is in thinking both trucks are in the same gear always, at the same time. They're not. The transmission gear is chosen based on throttle/load, and MPH.

When your truck is in 7th, mine will be in 6th. Doing the math, that means both trucks are putting the same force to the wheels.

OK I’ve lost interest in continuing. Peace out.

pacofortacos · Sep 9, 2022

ramffml said:
Why would that be the case when both trucks would be running at the same RPM at the same speed making the same HP/Torque when approaching that hill? There is literally no difference, some simple math to illustrate:

2 (transmission gear ratio) * 4 (rear gear ratio) = 8 (wheel gear ratio)
4 (transmission gear ratio) * 2 (rear gear ratio) = 8 (wheel gear ratio)

Doesn't matter which way you swap those, you end up with 8 at the end.

In real life it's actually something like:
1 (6th gear in tranny) * 3.21 (rear gear ratio) = 3.21 (wheel gear ratio)
0.82 (7th gear in tranny) * 3.92 (rear gear ratio) = 3.2144

Notice how both trucks end up with virtually identical wheel gear ratio when the 3.21 is in 6th vs the 3.92 in 7th.

Your rear gear ratio becomes irrelevant when you have tons of gears for the transmission to play with. If you had only 3 or 4 gears like trucks 20+ years ago, different story.

The actual ratio changes a bit below 6th gear between the two rear gear ratios.
What you state is true for 6th gear and up only.

3.21 vs 3.92 real world

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