Difference in gas mileage between the 3.21 and 3.92?

[Benca101]

I get 13.5 MPG on 87 octane and 14.5 to 15.2 on 89. One day, in the towing section, someone complained about their Rebel only getting 8 MPG towing a trailer. Seemed low.. the guy casually mentions setting the cruise control to 75 MPH with a large, 8000 pound trailer. I thought that was pretty funny.

Anyway, people complain constantly. A few days ago I filled up, reset my mileage, hauled *** onto the freeway and got stuck behind a truck pulling a trailer. He’s going 55 MPH. I turned on active cruise control and remained behind him for about 20 miles. Great tunes coupled with a phone call left me unconcerned about the speed. I pulled off some time later. Here is my mileage at the second red light from the freeway.

the pic sucks, but I was at 23.3 MPG after getting onto the freeway in a hurry, 2 red lights, 20 miles, and 1 long enormous hill. In a Rebel for god sakes.

 

[Scram1500]

I'm using the brakes a lot less than my previous gen with the 3.21. The 3.92 slows you down by just lifting your foot. The money I'm saving on brakes I can **** into the gas tank

 

[WXman]

RPM is only a fraction of the equation. Tires, load, terrain, how often your trans. is shifting, weather, regional fuel production differences, etc. all make a difference. It's virtually impossible to compare 3.21 to 3.92 on an internet forum.

I'll tell you this though, there is no question that 3.92 axle gears help the trans. stay in gear and downshift/upshift less, which takes a lot of wear and tear off the hardware and can actually lead to better MPG in areas like I live in here in the eastern U.S. Deeper gearing actually helps unless you live in Kansas.

One time, I had a Jeep with a manual trans that actually gained hand calculated MPG after I regeared the axles to a deeper ratio. The less the engine has to work, the more fuel you save. The engineers who claim 3.21s are better for MPGs are testing in ideal world conditions and usually in a laboratory.

 

[Gmoosevt]

There are many factors involved, yes, but one plain and simple fact (i.e. the truth): for normal driving, with two identical unmodified factory trucks with the 5.7: the 3.21 will get better gas mileage than a 3.92. Period. This is for the +90% of drivers who have the common mix of ‘in town’, highway, and towing once-in-a-while. That is, it applies to most of us DT owners. If you tow heavy stuff a lot, or off-road and like to be at a higher rev range when you juice it - then get a 3.92. If you like modding your truck with lifts, bigger tires, re-tunes, disengaging the MDS and so on, then mpg’s can’t be too high on your list of concerns, so get whatever rear gears trip your trigger. It’s pretty basic and simple: if you tow heavy loads, or over 4000lbs fairly frequently, then you probably want a 3.92. If not, you can (and will) get slightly better gas mileage with a 3.21. Several people in this thread like to espouse their engineering prowess of the 50 factors that will actually affect gas mileage - fine - but your are just complicating and confusing a simple bottom-line answer that applies to the 90% of Ram DT owners who are asking a simple question, and need a 20 second answer that makes sense to them, so they can make a good decision. An engineering lesson on energy, friction, thermodynamics, heat transfer, combustion, etc might be best left for a different thread.


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[Willwork4truck]

There are many factors involved, yes, but one plain and simple fact (i.e. the truth): for normal driving, with two identical unmodified factory trucks with the 5.7: the 3.21 will get better gas mileage than a 3.92. Period. This is for the +90% of drivers who have the common mix of ‘in town’, highway, and towing once-in-a-while. That is, it applies to most of us DT owners. If you tow heavy stuff a lot, or off-road and like to be at a higher rev range when you juice it - then get a 3.92. If you like modding your truck with lifts, bigger tires, re-tunes, disengaging the MDS and so on, then mpg’s can’t be too high on your list of concerns, so get whatever rear gears trip your trigger. It’s pretty basic and simple: if you tow heavy loads, or over 4000lbs fairly frequently, then you probably want a 3.92. If not, you can (and will) get slightly better gas mileage with a 3.21. Several people in this thread like to espouse their engineering prowess of the 50 factors that will actually affect gas mileage - fine - but your are just complicating and confusing a simple bottom-line answer that applies to the 90% of Ram DT owners who are asking a simple question, and need a 20 second answer that makes sense to them, so they can make a good decision. An engineering lesson on energy, friction, thermodynamics, heat transfer, combustion, etc might be best left for a different thread.


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Thats me, the simple answer. Now my engineer son-in-law, can’t get him to shut up about variable x and constant y... no wonder I don’t visit much.

 

[zelmo]

What? Another old fart on the forum? You mean my 1966 Pontiac Catalina wasn’t supposed to pull a 2 horse trailer with 2 horses in it?

When I was young and dumb I put a hitch on a '61 Falcon. The first time pulling the boat I ran through a red light because I couldn't stop in time.

 

[Willwork4truck]

When I was young and dumb I put a hitch on a '61 Falcon. The first time pulling the boat I ran through a red light because I couldn't stop in time.

Ha, for sure. i’ve seen vw bugs with hitches too... ah the things we used to do...

 

[Cravenfan]

About 25 threads on this topic, but it's still only about $1 per day, on average, more for the 3.92 gears, due to real or perceived gas mileage differences...so, there's that.

Ps - Your mileage may vary...

 

[zelmo]

Ha, for sure. i’ve seen vw bugs with hitches too... ah the things we used to do...

The first one looks staged but the second does not.

 

[Scram1500]

I have a small hitch on my Rokon, only been brave enough to move the wood splitter lol

 

[Gmoosevt]

About 25 threads on this topic, but it's still only about $1 per day, on average, more for the 3.92 gears, due to real or perceived gas mileage differences...so, there's that.

Ps - Your mileage may vary...

You must be rich, or bad at math. A dollar a day is $3650 over a 10 year life of the vehicle, in nominal dollars (although gas is ‘cheaper’ about now). With $3650 I could buy 10 face shields, a hundred masks, and maybe bribe a doctor for a 12-hour ventilator session. A 3.21 rear gear could save lives?


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[Willwork4truck]

You must be rich, or bad at math. A dollar a day is $3650 over a 10 year life of the vehicle, in nominal dollars (although gas is ‘cheaper’ about now). With $3650 I could buy 10 face shields, a hundred masks, and maybe bribe a doctor for a 12-hour ventilator session. A 3.21 rear gear could save lives?


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True however people waste way more than that on daily coffee stops at McD’s or Dunkin or Starbucks... and they don’t care.
For the power bump and faster acceleration, $1 a day is cheap if you are into that or need it for towing.
(I’m not but many are.)

 

[silver billet]

RPM is only a fraction of the equation. Tires, load, terrain, how often your trans. is shifting, weather, regional fuel production differences, etc. all make a difference. It's virtually impossible to compare 3.21 to 3.92 on an internet forum.

I'll tell you this though, there is no question that 3.92 axle gears help the trans. stay in gear and downshift/upshift less, which takes a lot of wear and tear off the hardware and can actually lead to better MPG in areas like I live in here in the eastern U.S. Deeper gearing actually helps unless you live in Kansas.

One time, I had a Jeep with a manual trans that actually gained hand calculated MPG after I regeared the axles to a deeper ratio. The less the engine has to work, the more fuel you save. The engineers who claim 3.21s are better for MPGs are testing in ideal world conditions and usually in a laboratory.

We've been through this before. Your 8th = my 7th, if I want to prevent lots of shifting I lock out to 7th or 6th while towing. When I'm not towing, my truck doesn't downshift/upshift anymore than yours in 99% of driving situations. Maybe in a few steeper hills at highway speeds, you might be lucky to find the right hill where my truck has to downshift but yours doesn't. Big deal.

Shifting may cause a tiny bit of wear and tear, but these are 8 speeds designed to be used as such. And you didn't account for the fact that your truck will spend it's lifetime revving higher than mine (so my tranny might have a bit more wear and tear due to a few extra shifts, but your engine will have more wear and tear especially if you do lots of 8th gear driving).

As for rear world testing conditions; every high mpg stat I've seen posted here has been in the 3.21. My personal best is 24 mpg on a 3 hour trip.

 

[WXman]

We've been through this before. Your 8th = my 7th, if I want to prevent lots of shifting I lock out to 7th or 6th while towing. When I'm not towing, my truck doesn't downshift/upshift anymore than yours in 99% of driving situations. Maybe in a few steeper hills at highway speeds, you might be lucky to find the right hill where my truck has to downshift but yours doesn't. Big deal.

Shifting may cause a tiny bit of wear and tear, but these are 8 speeds designed to be used as such. And you didn't account for the fact that your truck will spend it's lifetime revving higher than mine (so my tranny might have a bit more wear and tear due to a few extra shifts, but your engine will have more wear and tear especially if you do lots of 8th gear driving).

As for rear world testing conditions; every high mpg stat I've seen posted here has been in the 3.21. My personal best is 24 mpg on a 3 hour trip.

By your example, the 3.92 won't rev higher anywhere except high speed interstate driving, so there wouldn't be any additional wear of significance.

But, as we discussed when we went through this before, the 3.92 gear gets the truck off the line more easily and due to torque multiplication, not straight math, and actually allows the truck to stay in lower RPMs more often....reducing fuel consumption in most real world scenarios and reducing overall wear on the engine.

There's a reason the 3.92 gear is a $95 option and is one of the most frequently selected options on the order sheet.

Also, my 8th is not your 7th.

Gear x3.21 x3.92
5.00 16.05 19.60
3.20 10.27 12.54
2.14 6.87 8.39
1.72 5.52 6.74
1.31 4.20 5.14
1.00 3.21 3.92
0.82 2.63 3.21
0.64 2.05 2.51

As you can see there isn't one single place anywhere in the entire gear spread where the two axle ratios overlap. But, the 3.92 will be able to hold 8th far easier on the interstate, while still turning less RPM than 7th gear with 3.21 axle gears. So, the 3.92 is better everywhere from 0 to 80 miles per hour.

 

[silver billet]

By your example, the 3.92 won't rev higher anywhere except high speed interstate driving, so there wouldn't be any additional wear of significance.

But, as we discussed when we went through this before, the 3.92 gear gets the truck off the line more easily and due to torque multiplication, not straight math, and actually allows the truck to stay in lower RPMs more often....reducing fuel consumption in most real world scenarios and reducing overall wear on the engine.

There's a reason the 3.92 gear is a $95 option and is one of the most frequently selected options on the order sheet.

Also, my 8th is not your 7th.

Gear x3.21 x3.92
5.00 16.05 19.60
3.20 10.27 12.54
2.14 6.87 8.39
1.72 5.52 6.74
1.31 4.20 5.14
1.00 3.21 3.92
0.82 2.63 3.21
0.64 2.05 2.51

As you can see there isn't one single place anywhere in the entire gear spread where the two axle ratios overlap. But, the 3.92 will be able to hold 8th far easier on the interstate, while still turning less RPM than 7th gear with 3.21 axle gears. So, the 3.92 is better everywhere from 0 to 80 miles per hour.

Your math is wrong. See here:
https://5thgenrams.com/community/th...to-3-21-vs-3-92-axle-ratio.16986/#post-231619

Also your logic is wrong (the price difference of 3.21 vs 3.92 has no bearing on the discussion unless those of us with the 3.21 are somehow trying to claim the 3.21 saves you money because it's cheaper to buy; which, none of us ever claimed). The frequency with which a feature is selected (popular = better) is also not logically correct; otherwise "popular = better" should mean we should all be driving f150's.

 

[Scram1500]

Whew, almost as heated as a catch can topic. There are advantages and disadvantages to both, one is not better than the other. It's just personal preference. Simply put, top end vs acceleration, does one like to save gas or melt the tires? Also, isn't there a **** waving contest already about the 3.21 vs 3.92 stickied at the top of this page?

 

[Boston]

If this stupid C19 hadn’t shut down the dealerships and the need to travel, I’d be out there trying to close a deal on a 3.21 because 90% of my driving will be without a trailer.

 

[silver billet]

Whew, almost as heated as a catch can topic. There are advantages and disadvantages to both, one is not better than the other. It's just personal preference. Simply put, top end vs acceleration, does one like to save gas or melt the tires? Also, isn't there a **** waving contest already about the 3.21 vs 3.92 stickied at the top of this page?

Well I apologize if I've been acting like a ******, but that's just my lack of people skills showing up. Certainly not intentional to flame anyone, I just don't like misrepresenting/misunderstanding facts. This particular discussion (gear ratio) is entirely objective, it's just numbers and understanding how they work. My intention is just to visualize how it works, so that we can all feel better about our purchase.

Here is a graphic of a spreadsheet I built which clearly shows how the final gear ratios work. And while there is not an exact (down to 100'th decimal place) match between gears in these ratios, as @jake explained in his excellent thread "An engineer's ultimate guide..." they are close enough so that you would not notice the difference. The 3.92 has more off the line grunt, the 3.21 has an additional gear for highway speeds unloaded. However once you're up and running towing a trailer and you're past 3'rd gear, both trucks will have the exact same (for all intents and purposes) gears available so that each truck will feel just as powerful on the freeway while towing at highway speeds. I color coded the matching gears, and then also added a "difference" column which shows how little difference there is between the 2.

 

[Gmoosevt]

Wanderer... you seem to be mired in justifying your choice of the 3.92 gear rear. That’s fine. Glad you like it. To each his own. We all tow different stuff at different frequencies, and maybe some want to get off the line quicker than others. However, I might be wrong, but I think the original question was ‘which one gets the best gas mileage?’ The answer to that is simple, either by math, by logic, or by real world testing for the average driver that only tows his boats or trailers every once in a while: the 3.21 gear gets better gas mileage.... as attested to by many, many owners on this very forum (i.e. the ‘real world’). I haven’t heard any Rebel owners (all 3.92 gears of course) bragging about the gas mileage they get.... albeit they have moderately bigger tires and are an inch higher - which kills mpg’s as well. Secondly, why do you thing FCA chose the 3.21 as the standard rear for the truck that all the EPA mpg testing was done on? (that ‘logic’ answer is pretty obvious). And finally, from the ‘math’ perspective, one only needs to think about two identical trucks - but one with 3.92 gear set, and one with 3.21 - driving 55 mph on a flat highway, in identical conditions. Both will be in 8th gear (in identical trannys). The 3.21 will get better mileage by the predominance of lower friction losses from the engine running at lower rpm. Sure, there are many other factors involved, but engine friction is the dominating difference, followed probably by tranny fluid pressure and flow losses, followed by rear gear friction. There are 20 other friction losses involved as well, but almost all of those will be identical for the identical trucks and driving conditions: air friction and rolling mass friction being the biggest. But the point is: it is not debatable - for everyday driving - mixing urban, rural, highway, and occasional towing - the 3.21 is going to deliver better average mpg. Will it tow an 8000 lb horse trailer through the Catskills or over the Rockies as well? Nope. Will it beat the 3.92 in a quarter mile? probably not. Is it better for a weekend in Moab? Nope. So, if off-reading, heavy towing, and hole-shots are important to you, than luckily there is a 3.92 gear set to fit your needs.... but please don’t argue that it gets just as good gas mileage- because it doesn’t.


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[RangerZ]

My rebel isn’t my DD and I certainly didn’t buy it for MPG but it’s a gas piggy. Worse than any of my last 7 trucks. 8 if you include my AWD Yukon. It’s bad when it’s not towing and it’s really bad when towing But I love it. Finally installed my hellwig sway bar today so I like it even more now