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Switching from 3.92 to 3.21...

Edwards

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I was told that getting the lower gears (3.92) would allow the truck to switch into economy mode (go from 8 cyl to 4 cyl) easier and more often, as the load on the engine is lighter with the lower gears, so you actually would save fuel with the lower gears by being in 4 cyl mode more often. Can't vouch for that personally though. Just another factor to consider in all of this.

I've seen this as mine switches into ECO very often and has done it since 21 miles.
 

Jaysond

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Mine with 3.21 gears very rarely goes into econ mode, you almost have to be going down hill with the wind for it to switch. Normal highway driving on a flat road, even when going in the 55 - 60 mph range, it won't switch. Have been wishing I had the lower gears to hopefully save gas... again, no real world testing has been done by me comparing 2 equal trucks with the two different gear ratios to prove anything.

When I got my roll up box cover, I really thought that would help it switch more often, but it really didn't seem to make a difference. I have the Off Road Pkg with the extra 1" lift, and the more aggressive tires, I suppose both of those things add resistance at highway speeds.
 

lowell3170

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I was told that getting the lower gears (3.92) would allow the truck to switch into economy mode (go from 8 cyl to 4 cyl) easier and more often, as the load on the engine is lighter with the lower gears, so you actually would save fuel with the lower gears by being in 4 cyl mode more often. Can't vouch for that personally though. Just another factor to consider in all of this.
But you better say under 45 mph with those 3.92. You will get better mileage with the 3.21 all day long. As the RPM go up your MPG will go down.
 

Cannonball

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But you better say under 45 mph with those 3.92. You will get better mileage with the 3.21 all day long. As the RPM go up your MPG will go down.
Not necessarily.

W = F* D

I agree the engine will need to do more work over the same distance due to the resulting linear distance being less per shaft rotation. However, the 3.21 will always have to counter a slightly greater resistant force regardless of the distance (therefore work will also increase) This is due to more torque needed to translate the rotation of the small diameter drive shaft to that of the tire.

So we all are assuming the loss of linear distance outweighs the increase in resistive force. It very well may BUT if the reductution of force is enough to drop to 4 cylinders, THAN just maybe certain builds of trucks (weight, aerodynamics, tire weight and size) would actually benefit from the 3.92 at certain highway speeds.

Another example is loading up the engine when towing. Real world scenario, what ratio gets better mileage towing? I believe the 3.92 Because it can do work with less force albeit for 21.5% more engine rotations to travel the same distance.

Not fact, just a perspective.
 

Mopar21222

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Don't fear the gear, I've swapped gears in 2 trucks before, figure at least $1200-$1500 per axle. 3.21 vs 3.91 is basically 350-500 rpm difference in the same gear at the same speed. With the 8speed, you are basically going 1 gear in the tranny lower or higher with the respective axle ratio.

With the '03-'08 trucks with the 5spd and 3.92, you'd wind 1st gear out to 40-45mph to get to higher RPM, which drove me nuts, then you'd shift quickly thru the upper gears. This 1st spread by the numbers is essentially the 8-sp with 3.21s.

If you were in flat country, with mostly highway, 3.21 would be the way to go. If you upsize tires, which I suspect many will, as the stocks are 1 size too small IMO, thats like lowering the ratio even further.
 

ExcursionDiesel

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My Limited Etorque 4WD w/Off Road gets 20-21 instantanious mpg on the dash at 65 mph cruise on flat ground with 42 psi in the tires. Ride height is in Aero. I'm going to try 45 psi tomorrow. The tires show max as 51 psi. Rolling resistance plays a factor on how often ECO/MDS engages. Wind drag can't be helped.
 

Dadddeuce

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Maybe 1-2 mpg, depending on if you do mostly city highway...

Acceleration and towing of 3.92 is nice
It’s about a 4 mpg difference on high way my father-in-law’s 20 Laramie gets about 25-26 on interstate and 19 around town driving and my 20 bighorn with 3.92 gears gets like 13 around town and like 17 on the highway which is horrible I’m supposed to get 21 on interstate and 15-16 around town! I only tow a 3500lb bay boat, what was I thinking!
 

WXman

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I've had more trucks than I can count, with every ratio you can think of. There is NO WAY a change in axle ratio is going to equate to 3-5 MPG. Ain't happnin'.

I'd believe some folks could see 1 MPG difference, some nothing.
 

Timeless

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Might have been said, but I suspect the cost to do this would never catch up to what you would save...unless you kept the vehicle for a long time/many miles.
 

BowDown

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I've had more trucks than I can count, with every ratio you can think of. There is NO WAY a change in axle ratio is going to equate to 3-5 MPG. Ain't happnin'.

I'd believe some folks could see 1 MPG difference, some nothing.

Do the math, its a 20% difference in the gear ratio, 20% lower rpm and about a 20% mpg increase, thats about 4-5 mpg
2016x980.jpeg

Mostly city driving with 3:21 gears, probably 80% city
 

WXman

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Do the math, its a 20% difference in the gear ratio, 20% lower rpm and about a 20% mpg increase, thats about 4-5 mpg
View attachment 66283

Mostly city driving with 3:21 gears, probably 80% city

If only it worked that way! :D

Unfortunately, RPM doesn't translate to fuel usage in a 1:1 ratio. There are a thousand other variables.
 

BowDown

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If only it worked that way! :D

Unfortunately, RPM doesn't translate to fuel usage in a 1:1 ratio. There are a thousand other variables.

Lol, OK meanwhile 3:21 trucks are seeing 4-5 mpg on the highway
Here's a hint, Brake Specific Fuel Consumption
 

Timeless

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Lol, OK meanwhile 3:21 trucks are seeing 4-5 mpg on the highway
Here's a hint, Brake Specific Fuel Consumption

So how do you explain that the EPA does not see this in their certification?
 

BowDown

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So how do you explain that the EPA does not see this in their certification?

Its literally on the window sticker on both a 3:21 and 3:92 truck.
You have 3:92's are you getting 18+ mpg in the city? No.

Your logic states an engine running at 1500 rpm at 65mph consumes the same amount of fuel as an engine running at 2000 rpms at the same 65mph. Both fundamentally can't be true, you argue that the 20% difference makes no difference but provide zero facts to back that statement up and don't do the math its simple algebra.

All things, a truck operating at the same mph with a 20% reduction in rpm is going to consume less fuel yet you say no, this despite many providing so by displaying their mpg.

I had 3:92 in my last truck, I didn't do it again for a reason and that premise bore out to be proven factually sound in this truck. You like the 3:92s great but don't try to lie to others that it doesn't make a difference to try and justify your own choice.
 

Timeless

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Its literally on the window sticker on both a 3:21 and 3:92 truck.
You have 3:92's are you getting 18+ mpg in the city? No.

Your logic states an engine running at 1500 rpm at 65mph consumes the same amount of fuel as an engine running at 2000 rpms at the same 65mph. Both fundamentally can't be true, you argue that the 20% difference makes no difference but provide zero facts to back that statement up and don't do the math its simple algebra.

All things, a truck operating at the same mph with a 20% reduction in rpm is going to consume less fuel yet you say no, this despite many providing so by displaying their mpg.

I had 3:92 in my last truck, I didn't do it again for a reason and that premise bore out to be proven factually sound in this truck. You like the 3:92s great but don't try to lie to others that it doesn't make a difference to try and justify your own choice.

I think you have me confused with someone else as I am not arguing against you or posted any logic.

What I will post is two window stickers that don't seem to suggest you get 4-5 more MPG with 3.21 rear end.

2020 Ram Hemi 3.21.JPG

2020 Ram Hemi 3.92.JPG
 

BowDown

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My window sticker shows 17 and 23 for a 3:21. From what I've read, most with 3:21s are getting significantly better than the spread from 3:92 to 3:21 on the window sticker. I'm at 18 and change now and it keeps increasing, the 3:92 people that post are in the 12,13-14 mpg in the city, there's some posted in this thread. EPA rating is also a base for that model and in looking at the EPA website, optional equipment like gear upgrades are not factored in, they just the base model info, IE 4x4 4x2. I wouldnt base any mileage off EPA numbers anyway but real world numbers from people with those vehicles.
 

silver billet

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I think you have me confused with someone else as I am not arguing against you or posted any logic.

What I will post is two window stickers that don't seem to suggest you get 4-5 more MPG with 3.21 rear end.

View attachment 66500

View attachment 66501

You cannot use window sticker; EPA has the same rating for a tradesman as they do a rebel. Real world reports suggest the 3.21 can get at least 3 mpg more on the highway. In the city there is probably zero difference.
 
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turkeybird56

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How realistic / costly is it to switch a truck that comes with the 3.92 gear ratio back to a 3.21?

I’ve found several trucks that I like that have the 3.92 instead of 3.21, but I do not need the 3.92 as I do not do a lot of towing.

I realize this may be a really stupid question.

BOIRD confused? Why change? U got better towing and better rear end, higher load, just slightly lower MPG. And U got a 33 gallon fuel tank, a lot of PPL< especially getting the new E Torque rides with 23 gallon tanks would kill for a 33 gallon tank, all IMHO.......U R betta off with the 3.92, but we all have opinions, LOL......
 

2wd

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You cannot use window sticker; EPA has the same rating for a tradesman as they do a rebel. Real world reports suggest the 3.21 can get at least 3 mpg more on the highway. In the city there is probably zero difference.

This has been my experience. I routinely get 22-24 mpg highway w/3.21. City is likely no difference. Depending on your highway vs city miles you're saving a couple thousand dollars every 100,000 miles. Not worth it to do the swap IMO as it's pricey from what I've heard unless you're got 2WD
 

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