3:21 rear vs 3:92 gas mileage?

[MSDelta]

Hello,

I am looking to purchase either a new 5.7L Laramie or Limited. The issue is that the Limiteds I am looking at all have the 3:92 rear end. My usage will be 99% city and highway with no towing(at present).

Can anyone share their knowledge on the significance as it relates to gas mileage?

Thanks for your help!

 

[ramffml]

There is no difference in the city, you'll notice the savings only when the 3.21 is in 8th gear because at that point its RPMs are always lower than the 3.92 at the same speed.

In the city its a wash, both trucks will use the transmission to shift you where you need to be.

 

[MSDelta]

Thanks ramffml! Yes, I was thinking it probably would not be a big difference in dollars over a years time. Of course you would have to allow for gas price fluctuations which hopefully will be somewhat stable for a while.

 

[mikeru]

The biggest gain you'd see from having the 3.92 gear is starting from a dead stop, especially if towing moderate to heavy. And the biggest gain you'd see from the 3.21 is in 8th gear as @ramffml said already. People argue over which is better, but having owned both I can say that I wouldn't know which one I have if my window sticker didn't tell me LOL. Maybe if I towed or drove faster than 65 mph on a regular basis. Speed limits are low around here

 

[Ramsteen23]

I had 3.21 on my 2016 Limited and now have 3.92 on my 2023 Limited. No noticeable difference in MPGs...you will feel a little more giddy up from stop with the 3.92 and it handles the bigger/heavier tires I added very well but the 3.21 would perform just as well.

 

[BuschLatte420]

3.92’s are the way to go while america still allows it

 

[Jas34]

On my 2017 with .321 I usually see 22 on the highway and 15-17 around town. The newer ones with the etorque should do a little better (maybe 1-2 mpg?).

 

[TotallyHucked]

With these modern trucks, don't fear the gear. I have a friend with 3.21s and he averages ~17-19mpg. I have another friend with 3.55s and he averages 17-19mpg. My truck has 3.92s and on stock tires I averaged 17-19mpg.

In town, the 3.92s might get a smidge better mileage. On the hwy, 3.21s might get a smidge better mileage. But the 3.92s drive better IMO plus you can tow more.

 

[Stavinksi]

This for sure would be one of the last things on the feature list as the deciding factor for me as well. There isn’t enough difference either way. On a limited you probably hit payload, hitch, gvwr ratings issues before the 3.21 is your limiting factor. Likewise highway grades truck is shifting to 7th or maybe 6th anyways.

 

[ramffml]

In town, the 3.92s might get a smidge better mileage.

Nope. It'll be the same unless you construct a test that artificially runs the one truck at higher rpms for a very long time.

 

[TotallyHucked]

Nope. It'll be the same unless you construct a test that artificially runs the one truck at higher rpms for a very long time.

When I say smidge, I mean a smidge. AKA, not noticeable. For my heavy foot especially, there's no difference lol

 

[Wild one]

I run alot of 80+ mph roads,and going from 3.21's to 3.55's dropped my milege by a noticable amount,and also raised my oil temps to where i didn't like them,and i also gained a good 5 to 10 degree's in rear end heat.I kick my ass for going to the 3.55's,as my truck also didn't gain anything at the track.If you cruise freeways at 70+ mph,the 3.21's are a way better set of gears

 

[Wmjohn]

Say what you will but my 2024 Laramie 5.7 with 321 at 63mph is loafing along at 1400 rpm. Less engine wear, less heat. I know the tow guys like the higher gear and i see why.

 

[ramffml]

When I say smidge, I mean a smidge. AKA, not noticeable. For my heavy foot especially, there's no difference lol

I'm not trying to be argumentative, but lets look at the physics.

MPG is a function of RPMs, MPH, and load/resistance (lets ignore driver style). If MPH and load is the same between both trucks, then that leaves RPMs, which is determined by the transmission + gear ratio, but the transmission in both trucks will upshift you as fast as they can.

If you were to compare both trucks in the city at 30 mph, they will both be almost identical RPMs (give or take). Same thing at 20 mph. They will be in different gears, but the RPMs will be the same.

There are specific speeds where one truck will be at a slightly higher RPM, because the final gear ratios are not 100% equivalent. But then as the speed creeps up, the truck with the slightly higher RPM will upshift and now it's running lower while the other truck is higher and at that constant speed the first truck will do better. So it's a wash, unless you design an artificial test to keep the one truck at a higher RPM. In normal traffic, there won't be any difference as the speeds go up and down.

 

[ramffml]

Say what you will but my 2024 Laramie 5.7 with 321 at 63mph is loafing along at 1400 rpm. Less engine wear, less heat. I know the tow guys like the higher gear and i see why.

This "tow guy" loves my 3.21. My truck pulls my trailer in 6th gear (direct, no overdrive) and sits at about 2200 to 2300 RPMs for hours on end. What's not to like? In the city in first gear I can ram into traffic in front of me if I push too hard. I have absolutely zero issues pulling.

The 3.21 with 8 gears is a far stronger setup than the old 6 speed with 3.92 or even 4.30s.

 

[TotallyHucked]

I'm not trying to be argumentative, but lets look at the physics.

MPG is a function of RPMs, MPH, and load/resistance (lets ignore driver style). If MPH and load is the same between both trucks, then that leaves RPMs, which is determined by the transmission + gear ratio, but the transmission in both trucks will upshift you as fast as they can.

If you were to compare both trucks in the city at 30 mph, they will both be almost identical RPMs (give or take). Same thing at 20 mph. They will be in different gears, but the RPMs will be the same.

There are specific speeds where one truck will be at a slightly higher RPM, because the final gear ratios are not 100% equivalent. But then as the speed creeps up, the truck with the slightly higher RPM will upshift and now it's running lower while the other truck is higher and at that constant speed the first truck will do better. So it's a wash, unless you design an artificial test to keep the one truck at a higher RPM. In normal traffic, there won't be any difference as the speeds go up and down.

That's way too sciency lol

 

[bborzell]

‘22 Laramie eTorque w/ 3.92. Reliable 17-19 mpg all around average. 12-14 while towing camper with 6,000 GW.

 

[brian42]

I had the 3.21 and liked it. Even with the taller gears I only got about 15.5 mpg overall (60% highway) in my 5.7L non-eTorque. Subtract 1 mpg for winter blend.

Came from a F-250 diesel with 3.73 gears (8K lbs with me in the driver's seat) and it also got 15.5 mpg overall.

I did not tow much and did not have asiprations of lifts or bigger/heavier (E-rated) tires. If I did I would have definitely gone with the shorter 3.92 gear set.

 

[Tulecreeper]

I get 14 -15 MPG all around without towing, but most of my driving is at 40 MPH on country roads. When I do get the chance to get on the highway at 60-65 MPH, I get sometimes upwards of 25 MPG but that's like twice a month for an hour.

 

[Mil T]

Mileage is all about right foot pressure. The more pressure that right foot gives the less mileage. Me I have 3:92's and I like the get up and go so Mileage is not as good as an old mans persay. Oh yea, I forgot that I'm one of those old men but with a heavy right foot. LOL.