Switching from 3.92 to 3.21...

[Accountant]

OP have you tried one with 3,92 and one with 3.21? I did and felt that the 3.92 truck was more responsive. I avg 17 mpg with mixed hwy and city plus have added roughly 150 lbs of weight with Mopar off-road running board, Retrax bed cover and front tow hooks.

 

[knightro84]

Provided the tire size would be the same, in 8th gear (0.67 ratio) at 70mph with 3.92 gears you should be turning 1930 RPM at 70 mph. With 3.21's you'd be turning 1580 RPM.

 

[SpeedyV]

Provided the tire size would be the same, in 8th gear (0.67 ratio) at 70mph with 3.92 gears you should be turning 1930 RPM at 70 mph. With 3.21's you'd be turning 1580 RPM.

And even that calculation doesn’t tell the whole story. For example, the engine will be making more power at 1930 RPM - possibly enough to handle aerodynamic drag more efficiently than if it were spinning at 1580 RPM - thus offsetting some of the theoretical loss of the RPM difference. Right?

 

[knightro84]

And even that calculation doesn’t tell the whole story. For example, the engine will be making more power at 1930 RPM - possibly enough to handle aerodynamic drag more efficiently than if it were spinning at 1580 RPM - thus offsetting some of the theoretical loss of the RPM difference. Right?

I agree.

 

[19RamLimited]

I think it’s important that someone that has 3.92 confirm those numbers.

 

[202snowman]

I think it’s important that someone that has 3.92 confirm those numbers.

Who in the hell can ONLY go 70 mph?

I can try on my way home tonight

 

[knightro84]

I think it’s important that someone that has 3.92 confirm those numbers.

The 1930 number assumed a 32" tire, so anything larger would yield a lower RPM and anything smaller would yield a higher RPM - but the 1930 should be close for anyone on the stock 275/55/20 tires.

 

[knightro84]

Who in the hell can ONLY go 70 mph?

I can try on my way home tonight

If you want to do 80, it should be 2200.

 

[Rammer76]

Also, if you have a 4x4 you have to change the gears in the front as well $$$$$.

 

[Accountant]

Also, if you have a 4x4 you have to change the gears in the front as well $$$$$.

I heard it’s welded shut so the whole assembly needs to be replaced. Is that right? Definitely $$$$

 

[Phoon]

Gear changes were more of a game changer with old 3 speed or 4 speed autos, these new 8 speed transmissions have a huge ratio spread, so the exact rear end ratio isn't such a big deal. My old truck had the 3:21, but with the low first gear it felt much more aggressive than i would have thought. The new one has 3:92 gears and it hits harder off the line, how much is gearing and how much is the new calibration and lighter weight is hard to say. My opinion is that it isnt worth the $$ to change it over, it wont drive much different or get much different economy wise.

 

[Ramjack]

And even that calculation doesn’t tell the whole story. For example, the engine will be making more power at 1930 RPM - possibly enough to handle aerodynamic drag more efficiently than if it were spinning at 1580 RPM - thus offsetting some of the theoretical loss of the RPM difference. Right?

No. This is a common misconception. Both trucks are making the same horsepower at 70 mph regardless of rear end ratio. They are both making just enough horsepower to overcome the drag experienced at 69.99999mph. (Let's say 50hp, but I really have no idea and it does not matter.) Even though the 3.92 truck is at a higher rpm, you have a lower throttle load compared to the 3.21 truck. Since both trucks are making the same horsepower, the 3.21 truck at 1580 rpm is making more engine torque than the 3.92 truck at 1930 rpm. So the 3.21 truck is making its horsepower more efficiently. Another way to look at it: If a 3.92 truck runs 1930rpm in 8th gear at 70mph while a 3.21 truck runs 1930rpm in 7th gear at 70mph, they would be equally efficient, until the 3.21 truck shifted to 8th. Granted, it's hard to say how much difference there would be in efficiency but I suspect it would be significant at highway speed since the rear end ratio difference is nearly 20%. Around town it obviously gets more complicated and there is undoubtedly less benefit, but the 3.21 will still be more efficient.

BTW, if I buy, it will be a 3.92.

 

[Rammer76]

I heard it’s welded shut so the whole assembly needs to be replaced. Is that right? Definitely $$$$

I have not checked, but I believe it's a two piece case that comes in half sort of like a transfer case. I'll take a closer look next time I'm under the truck.

 

[SpeedyV]

Since both trucks are making the same horsepower, the 3.21 truck at 1580 rpm is making more engine torque than the 3.92 truck at 1930 rpm. So the 3.21 truck is making its horsepower more efficiently.

You correctly point out that the power output must be equal in this scenario. I was suggesting that this oversimplifies the equation, as it incorrectly presumes the specific fuel economy of the engine is constant across its RPM range (i.e. ignoring the characteristics of its powerband). That being said, even if the engine has a slightly higher efficiency ratio at a higher RPM, as it comes closer to peak torque output, this does not mean the engine consumes less fuel overall when spinning at a higher RPM. The presumption that a taller gear will achieve better economy for a given vehicle is generally valid as long as the engine remains within its powerband (e.g. not "lugging"). Is that fair to say?

 

[Cannonball]

My 2013 Ram 1500 V6 can pull a 18 foot boat with ease and 3.21 rears. I bought it new in 13 and have 55000 miles on it . My MPG in warm weather id 22.5 to 25.5 and have gotton 27.7 on a full tank run. That little motor will shock you when passing other vehicles. I did order a 2019 Laramie with the V6 in August and waiting for it to arrive. Those 3.92's will kill your MPG. for sure.

What cab, trim level, and drive is your truck?

 

[Ramjack]

You correctly point out that the power output must be equal in this scenario. I was suggesting that this oversimplifies the equation, as it incorrectly presumes the specific fuel economy of the engine is constant across its RPM range (i.e. ignoring the characteristics of its powerband). That being said, even if the engine has a slightly higher efficiency ratio at a higher RPM, as it comes closer to peak torque output, this does not mean the engine consumes less fuel overall when spinning at a higher RPM. The presumption that a taller gear will achieve better economy for a given vehicle is generally valid as long as the engine remains within its powerband (e.g. not "lugging"). Is that fair to say?

I agree partly. But my point was not to suggest that the specific fuel economy of the engine is constant across its RPM range; just the opposite in fact. If the truck requires 50hp to maintain 70mph (again, just a guess on my part; it doesn't matter), it will generate that power with the least amount of fuel at the rpm that requires the greatest throttle opening to generate and maintain 50hp (w/o exceeding 50hp). This minimizes "throttling losses"/"pumping losses". This is obviously a very theoretical example but it illustrates a key point. There are practical considerations, of course. For example, nobody would want a 2.00 rear end just because it results in better mpg. I guess what I'm trying to get at is: I don't believe there is any reasonable scenario (towing or not) where a 3.92 gear will achieve equal or better MPG than a 3.21.

 

[202snowman]

Who in the hell can ONLY go 70 mph?

I can try on my way home tonight

Here you go...22 inch tires..sorry for the clarity of the picture..right around 1850-1900 at 69 MPH, 3.92 gears and the rest you can see in my signature

 

[lowell3170]

What cab, trim level, and drive is your truck?

2013 SLT Quad Cab V6, 2 Wheel Drive.

Here you go...22 inch tires..sorry for the clarity of the picture..right around 1850-1900 at 69 MPH, 3.92 gears and the rest you can see in my signature


View attachment 6117

I have 3.21 rears, at 70 mph My RPM is at 1500 RPM.

 

[202snowman]

2013 SLT Quad Cab V6, 2 Wheel Drive.

I have 3.21 rears, at 70 mph My RPM is at 1500 RPM.

Which is around where it should be, although you don't state your tire size. If you get a chance, please put the details of your truck in your signature

 

[Jaysond]

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.