Differential Effect on MPH

[Dan Weaver]

I am considering buying a 2017 Ram 1500 with the 5.7, 8 speed tranny and 3.9 differential. I want the higher tow rating, but does anyone know how much lower the mpg will likey be compared to a 3.2 differential?

Thanks

 

[rzr6-4]

8 speed is pretty good all around. I would guess maybe 1 or 2 mpg difference if you do a lot of highway driving but in town it won't matter much. I would have no problem with the 3.92 diff.

 

[Dan Weaver]

That's what I was hoping to hear.

Thanks

 

[Curmudgeon]

I really like the 3.21 in my 2014, and at 70mph on the highway my
engine is turning a mere 1500rpm, nice and quiet, 8-speed.

But I don't tow. And I knew it when I bought it. I probably would
have wanted the 3.92 if I did.

 

[CanuckRam1313]

I am considering buying a 2017 Ram 1500 with the 5.7, 8 speed tranny and 3.9 differential. I want the higher tow rating, but does anyone know how much lower the mpg will likey be compared to a 3.2 differential?

Thanks

My (then purchased new) 19' with the HEMI had the 3.21's and the 32 gallon tank...
I was quite happy with the highway fuel economy which gave me (averaged out) 900Km's of driving on the highway.

My now (purchased new in Feb 24') Warlock with the HEMI has the 3.92's and the same 32 gallon tank...
I'll average 750Km's of driving on the highway.

The difference is night and day when it comes to how this truck with the 3.92's takes off from a dead stop... and hammers down when I need to pass on the highway.

However, the RPM's at typical cruising speed is drastically different.
3.21 @ 120KM/H = 1400 to 1500 RPM
3.92 @ 120KM/H = 2000 to 2100 RPM

This said... my transmission does not shift near as much at highway speeds with the 3.92 vs the 3.21... I can mildly accelerate with no downshift with the 3.92 vs the 3.21 shifting considerably more often in relation to the throttle input and load (going up a hill, etc.).

For me, I did not purchase a near 6,000lb truck (with 4x4) and a HEMI for fuel economy whatsoever, so that is not in any of my deciding metrics at all!

But, having both gearing options back-to-back... I'd not ever go 3.21 again as the more trans shifting with the 3.21 at highway speeds and throttle input, and.... the better standing take off of the 3.92 puts that smile on my face that just can't be removed.

Let alone that with the 3.92 I can tow 3,000lb more vs the 3.21... not that I tow heavy often, but having the ability to do so is another added bonus of having 3.92 gearing.

This is all just my opinion though.
You do what is best for you.
Regardless, you won't be disappointed with either the 3.21 or the 3.92.
Just choose what works best for you!

After my diatribe...lol
It's marginal at best the difference in fuel economy between the two gearing options.
But, you did indicate that a higher tow rating is important...so that said, go with the 3.92 and don't look back!!!
My $0.02!

EDIT ****

Being a used purchase, I would absolutely have the trans filter and fluid changed immediately!

I would also install the @caulk04 trans cooler bypass upgrade at this time, too (drops trans temps by up to 30 deg/c right off the hop!).

I would also look to install the upgraded PPE trans pan (holds much more fluid) that has a replaceable filter, too!

Don't forget to change out the front & rear diff fluids and the transfer case fluid as well (if it's 4x4) at the same time.

Start fresh and enjoy your ownership experience to the fullest!

 

[CanuckRam1313]

I really like the 3.21 in my 2014, and at 70mph on the highway my
engine is turning a mere 1500rpm, nice and quiet, 8-speed.

But I don't tow. And I knew it when I bought it. I probably would
have wanted the 3.92 if I did.

^^^^ THIS!!!!

 

[Bigskyroadglide]

Here ya go, 5.7, 8 speed 3.92. 2250 at 85

Then 5.7, 8 speed 3.21 at 1850 at 83

 

[Wild one]

The differance in rpm is pretty well 22%,as 3.21's are roughly 22% less then 3.92's

 

[Burla]

You said mph in title and mpg in text, I'm guessing you want to know mpg. You can look it up just conf since it is factory option.

tow chart

I would want 3.92's personally, but the 8 speed is a very different transmission then we were ever used to, it is a great equalizer to keep trucks in good rpm.

 

[crash68]

My (then purchased new) 19' with the HEMI had the 3.21's and the 32 gallon tank...
I was quite happy with the highway fuel economy which gave me (averaged out) 900Km's of driving on the highway.

I still say it's the spark plugs that are the fuel economy culprit...lol
With a 6 gallon smaller tank, I usually average closer to 600 miles(965 Km) even with 3.92 gearing. At 70 mph is just under 2K rpms and turbo is pushing about 10 psi of boost.

As for the OPs concerns, the 3.92 should net slightly better fuel economy if the truck is city driven more than highway. If towing is a concern the 3.92 is definitely the way to go, the rpms will naturally be higher with a trailer attached.

 

[Hanover Fiste]

I wish Ram would offer the 3:55 ratio for the Hemi engines.

 

[Wild one]

I wish Ram would offer the 3:55 ratio for the Hemi engines.

They did for about 6 months in 13,back when they first started using the 8 speed MY13,you could get a Hemi / 8 speed with 3.55's,but when the 14's came out,that option was no longer available.Personally i think they screwed up by dropping the 3.55's with the hemi/8 speed.

 

[cnovel]

2500 Cummins 2015 with 3.21 pullls my 13,000 trailer all over , mountains to plains , no problem , and 22mpg without trailer at 70mph, with the 68rfe

 

[ppine]

An 8 speed trans has at least 2 over drive gears. You can always shift into a lower gear.
For towing the 3.92 is probably best.
I don't really like the idea of loafing along at 1,500 rpms with a gas engine at highways speeds.

 

[Dusty]

My 2014 (2wd, Quad Cab) with 8-speed and 3.92 diff rendered 18.64 MPG lifetime.

My current 2019 with 8-speed and 3.21 diff has rendered 19.91 lifetime so far.

The 2019 (2wd, Quad Cab) with 3.21 handles my towing requirements of 7000 pounds just fine.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33-gallon fuel tank, 18” wheels. Build Date: 3 June 2018. Now at 133455 miles.

 

[knightwe]

I am considering buying a 2017 Ram 1500 with the 5.7, 8 speed tranny and 3.9 differential. I want the higher tow rating, but does anyone know how much lower the mpg will likey be compared to a 3.2 differential?

Thanks

I had a 2018 with a 3.2 at 70 the rpm 1600. My 2021 with the 3.9 is 1900 at 70. MPG 20.5 for both.

 

[06 Dodge]

2500 Cummins 2015 with 3.21 pullls my 13,000 trailer all over , mountains to plains , no problem , and 22mpg without trailer at 70mph, with the 68rfe

IIRC the 2015 pickups with Cummins changed to 3.42 from 3.73 or it could be ordered with 4.10

 

[Marshall]

I am considering buying a 2017 Ram 1500 with the 5.7, 8 speed tranny and 3.9 differential. I want the higher tow rating, but does anyone know how much lower the mpg will likey be compared to a 3.2 differential?

Thanks

that is what I have in my 2014 sport, I really like it, milage is great, though I don't tow any more.
The milage really changes when the foot goes down, but it would also do that with any ratio.
It you are towing heavy all the time, You should be looking at a 2500 with the diesel.

 

[TimboRam77]

3 limiteds, 1 with 3.21 and two with 3.92.
I did find the 3.21 more pleasant on interstate driving with a catch.
I've done an exhaustive search online comparing. It's always the 3 21 people wanting to switch to 3.92.
Imo the 3.92 is preferable on interstate.
The catch is again imo is that the 3.92 gets you above 2200 rpm on interstate which allows for more oil splash which MAY preserve your engine. Plus I love hearing more engine noise.
With that said, if I were driving mostly interstate unloaded then the 3.21 would be a consideration. Like constantly crossing TX with a 75 speed limit.

 

[warlock23]

They did for about 6 months in 13,back when they first started using the 8 speed MY13,you could get a Hemi / 8 speed with 3.55's,but when the 14's came out,that option was no longer available.Personally i think they screwed up by dropping the 3.55's with the hemi/8 speed.

I have a 2023 Warlockthat came with 3.55 differential and 17' tires Could someone tell me the Pros and Cons of this type of differential and did they put smaller tires because of the differential size ?
Thank you