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3.21 4x4 - going across OK/TX - 24 mpg at 80-85 mph.
At 75 I saw 31….
In particular you may find a good fuel additive that’s effective will help, particularly at higher RPMs where combustibility is more important.
 

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Crossposting from ramforum.com as I wanted more data points and insights: Regretting ordering 3.92 on my 22 Ram Ecodiesel and almost debating paying to downgrade to 3.21

Here's the gist of the post:

Hi, I had ordered my RAM with 3.92 ratio as I only seemed to come across the good articles at that time in which I learned that it will have a higher resale value in the future, better for towing, etc. I did see some mention of a negligible difference in mpg but thought nothing of it as the sticker did not update on the MPG at all after I selected 3.92. I had even picked up the bigger diesel tank because I wanted to be able to go on long trips while dreaming about the 29mpg thats advertised lmao.

I live in California, almost everyone and their mother drives 80mph here on the highways and most drive even faster. I live in a fairly rural area, I have to drive on the highways a fair bit in order to get most things.
In my first few weeks of testing, here's what I've noticed for mpg
80mph = 2200/2250 rpm and ~20mpg
75mph = Just over 2000 rpm, maybe 2050/2075 and ~24mpg
70mph = a hair under 2000 rpm maybe 1950 and ~27.5 mph

I did reach out to the service department at my local dealer and was told $2,750 + labor in order to switch to a 3.21 (guy sent me a detailed parts list as for the breakdown).

In terms of switching over
Pros:
For the most part I can drive as I normally do and not have to worry about being inefficient in terms of diesel
Would be saving time as well as I'd be driving at 80mph instead of 70/75

Cons:
The upfront cost of course
Potential resale value in the future may be impacted but I dont like switching cars very often
Towing impact, I dont tow much now but it was just futureproofing

You may be calling me crazy but this is bothering me so much, like how did I look into every little detail and end up making this big of a mistake. Also, I am under the assumption that going to 3.21 will give me lower RPM's at 80mph, please correct me if I'm wrong.



I did do some analysis on things after the comments in that original post, assuming that the swap ends up costing me 5k for parts and labor, it would take me over 50k miles (which would be at least a few years) to recuperate the costs of swapping. So I'm thinking it's not worth it but I wanted to get as much data as I could. Thank you,
Put on some taller tires instead if it's something you would do later an easy fix to a expensive problem. You can always sell your tires
 

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2019 RAM 1500 Classic Tradesman 4X2 EcoDiesel
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I live in California, almost everyone and their mother drives 80mph here on the highways and most drive even faster. I live in a fairly rural area, I have to drive on the highways a fair bit in order to get most things.
In my first few weeks of testing, here's what I've noticed for mpg
80mph = 2200/2250 rpm and ~20mpg
75mph = Just over 2000 rpm, maybe 2050/2075 and ~24mpg
70mph = a hair under 2000 rpm maybe 1950 and ~27.5 mph
Many in CA drive the speed limit, 65 or 70 MPH on freeways. The fact that others don't care how much they spend on fuel does not obligate you to burn extra fuel. The simple guess is that you like to drive fast and use the behavior of others as a rationale.
 

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I have a 20 ED with 3.21 and pull a 7k camper. I see no need for the 3.92 and I have another 2k trip in a month pulling it. Avg 25mpg city/highway to work and 14mpg with camper. I pull camper around 68-72mph. Weekly driving is a lot of stop n go.
I think the point is the money to change the gears is not worth it. You might see a slight increase in economy but would never recoup the cost of the gear change.
 

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I would keep what you have but if you just have to do it, go to 3.55. It can still pull a trailer but have a lower rpm at highway speed. In the olden days, we added an overdrive unit. With all the RV's out there, surely they have an overdrive unit for that transmission. Then you have the best of both worlds.
 

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2021 Ram 2500 Tradesman Crew Cab 6.7 L Turbo Cummins 3.73
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Switching from the factory 32” diameter tire (275/55R20) to a 34” tire (275/65R20) would change your effective rear gear ratio from 3.92 to 3.67.
Bingo we have a winner!

Yea, bigger tires, you can have whatever ratio you want.
 

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Stock 2021 with 3.21, crew cab, 4x4, larger bed and fuel tank at 65-75 mph I get between 30 and 33 consistently. I love it.

Best case would've been to order it that way. You'll remember next time. Enjoy this one.
 

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OP, not sure how many miles you have on your new EcoD, but I'd say try not to take the MPG figures too hard until the truck is truly broken in. I mean at least 30-40k miles. I know the manual says the engine is broken in at the factory, but in reality the true break-in occurs with time and mileage.

In the meantime, try to slow down. These RAMs are flying bricks, they're not slipstream hypermilers. Being in CA, you probably already have some of the best D2 fuel available, but I've noticed that a small bump in cetane helps with the fuel economy numbers.

Lastly, I haven't paid too much attention to the RPMs in 8th gear doing 80 mph, but I will. My '22 EcoD has the 3.21 rear end, so I'll be able to tell you exactly what the numbers are. I really don't think it'd be worth the cost either way.

Edit: have you checked your tire PSI? Chances are they could be a bit higher. I run 40psi all around and see 28-30mpg on the interstates with the cruise set at 80.
 
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You guys that are going on what the dash says for mileage, well, I log every fill-up. Over 72K miles in my '19 Hemi, it was always high, but by random amounts. I mean, very occasionally, it would be spot-on.

On my '21 ED, I have now put a bit over 2000 miles on it. Bought it used with 17K on it, so it's reasonably well broken-in.

Fill ups:
524 miles, actual: 26.7, dash: 28.1 <-- 1.4 MPG high (all Interstate, with cruise set at 80, generally)
403 miles, actual: 24.1, dash: 26.4 <-- 2.3 MPG high (all Interstate, with cruise set at 80, generally)
199 miles, actual: 19.4, dash: 20.3 <-- 0.9 MPG high (half city, half highway)
488 miles, actual: 24.9, dash: 28.1 <-- the dash was high by over 3 MPG! (all Interstate, with cruise set at 80, generally)
551 miles, actual: 22.3, dash: 22.5 <-- by far, the closest it has been, but still 0.2 MPG high (something like 80% highway)

This is with stock wheels and tires.

You can't dismiss it as being "the way I filled up" or anything like that because it's ALWAYS high. If it was the way I fill up or differences in the pump cutting off or how level the truck is, then some would be high and some low.

One fill-up, the pump might cut off early, so I log that I used less fuel and my MPG are high. But, that means the next time I fill up, I will have started with a tank that wasn't totally full, so when I fill it up then, it will take extra fuel and the MPG would be low.

When it is always high, every single fill-up, then it is not the pumps or the way I fill up. It is the dash computer being high.
 

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2021 Ram 2500 Tradesman Crew Cab 6.7 L Turbo Cummins 3.73
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Oh, Oh the Lie-O-Meter!

I had a 2017 Ecodiesel, and it was mostly very generous on the dash EVIC display. My new 2021 2500 Cummins is even worse by up to 3 mpg too optimistic.

I don't know why I look at it anymore.

I hand calculate every tank fill. It's pretty easy when you have the odometer, a smart phone in your pocket, and the gallons to your left.
 

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Yes, the dash MPG display lies. It's 1-3 MPG optimistic at all times.

Yes, taller tires change your effective ratio, but at the price of lower MPG. Leverage against the brakes and driveline means you're going to be spending more on fuel. I ran 295/60R20 on my EcoDiesel and had a 20-21 MPG average over the course of a year. It's a substantial loss when you upsize tires.

The guy who said you'll never recoup the cost of a gear swap at the fuel pump is spot on. It would take over 8 years to possibly break even.

Having said all that, I've always had 3.92 gear in my Rams in the past. This new '22 I picked up has the 3.21 and I really don't like them. It's a very different driving experience and the towing rating is pathetic on this one. 8,150 by the VIN when I check the Ram site. There are midsize trucks close to that.
 
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If you realy want to do the change $2750 for parts and labor is cheap jump on it.

on the other hand the decrease in mileage is part gearing part wind resistance of traveling faster, I’m not sure your going to see a huge difference in economy after making a gearing change.
I had the same concern but I ended up with the 3.92 and was very happy. And if you're only concern is mileage $4000 buys a lot of fuel!
 

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Reporting back with the revs @ 80 mph: running 40psi all around, I'm seeing 1800-1850 rpms @ 80mph indicated.
 

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I don't think you will see a large jump in economy. 80 mpg will net you low mileage in a truck. Just the reality of aerodynamics
Best mileage I've ever gotten is by keeping speed around 65. Netted me 27 mpg. Driving at 80, nets me around 23 mpg. Both tanks were all highway with refill around 500 miles.
 

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Best mileage I've ever gotten is by keeping speed around 65. Netted me 27 mpg. Driving at 80, nets me around 23 mpg. Both tanks were all highway with refill around 500 miles.
I'm guessing you have a 3.92 rear end?

I have a 3.21 and I have gotten almost 27 MPG on a couple of different all-highway runs with the cruise set at 80. And, those were actually the best mileage I've gotten, period. But, I haven't had any runs yet where I was on reasonably flat highways and cruising at 60 - 70. I've only put 2800 miles on my truck since I bought it (but I bought it used, with 17K on it, so it's not new).
 

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I'm guessing you have a 3.92 rear end?
@ncskibum has a 2nd gen EcoDiesel and you have a 3rd gen EcoDiesel. The 3.55 gears in the 2nd gen EcoDiesel are similar to the 3.21 gears in the 2020+ trucks.
 
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2019 Ram 2500 Cummins - EX Ecodiesel owner.
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MUST hand calculate. M Ecodiesel was optimistic. This Cummins is also, maybe not so dramatic but still optimistic.

Last fill-up had zero towing but lots of urban crawl and idling. Showed 21.7 mpg. Reality calculated was 20.0mpg. Pretty typical.
 

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I think if you don't write it down, dash and hand-calculation, at every fill-up it is really easy to end up believing that "it's pretty close" when it is really not. For example, you see 24.something on the dash and you hand calculate and get 23.something and tell yourself, "yep, it was about right." But, you don't really pay attention that the dash said 24.7 and the hand calc said 23.1, so it was really 1.6 MPG off. And, once you sort of mentally dismiss that as "pretty close", then you also don't really notice that the dash is higher than actual pretty much every single time.

I've only had 8 fill-ups in my ED, so far. It was actually low on the dash one time. It said 24.5 and it was actually 25.4.

Every other fill-up, the dash display was higher than actual. One time, it was off by OVER 3 MPG. Another time, high by 2.4 MPG. And another by 2.3 MPG.

Posts about mileage that are based on what the dash display says are really pretty much useless.

My '19 Hemi was the same. Not the same numbers, obviously, but always off, and pretty much always higher on the dash display than actual, and the amount it was off would vary from almost accurate to 2 MPG or more.
 
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