An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[bwsRam19]

No it doesn't, there's probably 7 hp difference between 1500 and 2000 rpm and the HP made at either RPM isn't necessary to keep the truck moving at that speed. Throw in some hills then yes but both are downshifting at that point.

You keep saying it makes 7 more hp, which is what i keep saying, and then you say im wrong.

 

[Scram1500]

 

[BowDown]

You keep saying it makes 7 more hp, which is what i keep saying, and then you say im wrong. Oh yeah go **** yourself, now i'm done.

And you keep saying it'll work harder and use more fuel, it doesn't.
You think 7 hp is significant? Its not. And you never said what the hp difference was, you likely haven't a clue.
The RPM required to turn the rear gear faster to achieve a specific mph is. You're done again? good nothing but a forum troll that clearly doesn't understand physics or BSFC

 

[SpeedyV]

Surely I'm not the only one that realizes dyno charts are irrelevant to this discussion...right?

@BowDown and @bwsRam19 have been arguing a steady state problem, as defined by @Rossum in post #360. May I suggest some background reading on the law of conservation of energy, followed by the concept of mechanical advantage (the latter of which is nicely covered in this discussion on gear ratios).

As you were

 

[d15nonvtec]

Jesus........does the butt dyno count here? i had a 19 Laramie 5.7 with 3.21 gears and moved into a 21 Limited with 3.92 gears. feels pretty similar.

 

[Buz]

I'm getting convinced the 3.92 is Ram's way of pleasing all the folks that intend to lift their trucks and put larger tires on them.
I believe THAT is the main reason why they still offer it.

 

[cipherbreak]

I got the 3.92 gears specifically because I was planning on putting on larger tires. At the end of the day, I couldn’t care less what other people have.

 

[neeginan]

I picked up a 2020 Ram 1500 Big Horn Crew Cab 4x4 with the 5.7 eTorque and the 3.92. Drove from Atlanta to South Florida and got 19.8 mpg highway driving at between 75 and 80 MPH. Way better than what I expected. Hell, my wifes 2012 Nissan Pathfinder 4x4 with a 4.0L V6 only gets about 17 MPH under the same conditions. I expect the number will drop a bit when I put larger AT tires on, but still. I don't buy trucks for their fuel economy, but color me impressed by this one's so far.

 

[Hydroblueguy]

Jesus........does the butt dyno count here? i had a 19 Laramie 5.7 with 3.21 gears and moved into a 21 Limited with 3.92 gears. feels pretty similar.

I’m going from a 19 with 3.92 to 21 3:21! I love the performance of the 19. I hope I don’t notice much difference!

 

[rrbhokies]

I’m going from a 19 with 3.92 to 21 3:21! I love the performance of the 19. I hope I don’t notice much difference!

I'm looking at the same thing! I had a 19 with the 3.92 and standard v8 Hemi and got about 12.9mpg around town (back and forth to commuter lot, running errands, etc.). I enjoyed how fast off the line it was for a heavy truck. The 21 I'm looking at has the 3.21 ratio and the eTorque engine. I've been following this thread. I'm hoping for slightly better fuel economy from the 3.21 and eTorque (at least anything better than 12.9mpg would be great), but hoping that when I put the foot to the pedal from a stop light, the truck behaves much like my 19 did and doesn't behave like a Chevy Chevette off the line.

 

[z0n3]

I'm looking at the same thing! I had a 19 with the 3.92 and standard v8 Hemi and got about 12.9mpg around town (back and forth to commuter lot, running errands, etc.). I enjoyed how fast off the line it was for a heavy truck. The 21 I'm looking at has the 3.21 ratio and the eTorque engine. I've been following this thread. I'm hoping for slightly better fuel economy from the 3.21 and eTorque (at least anything better than 12.9mpg would be great), but hoping that when I put the foot to the pedal from a stop light, the truck behaves much like my 19 did and doesn't behave like a Chevy Chevette off the line.

I test drove a 3.92 and own a 3.21 when choosing my gear ratio for my ordered 2021. I still chose the 3.21. The difference between the two was not enough for me to switch.

 

[BowDown]

eTorque and 3.21 rear @7000 miles
80/20 city to highway

 

[z0n3]

eTorque and 3.21 rear @7000 miles
80/20 city to highway

View attachment 74270

Nice mileage for that much city.

 

[OliveBranchRAM]

My gas mileage absolutely sucks on my '19 Limited w/ 3.92, but I'm having so much fun since adding the Pedal Commander/CAI/Corsa Sport Exhuast that I really could care less!!! This truck now behaves like I want it to.

 

[z0n3]

My gas mileage absolutely sucks on my '19 Limited w/ 3.92, but I'm having so much fun since adding the Pedal Commander/CAI/Corsa Sport Exhuast that I really could care less!!! This truck now behaves like I want it to.

Except for the fuel consumption it sounds like you’re enjoying it! What kind of mpg are you seeing?

 

[OliveBranchRAM]

Except for the fuel consumption it sounds like you’re enjoying it! What kind of mpg are you seeing?

If I drive "normally" I am seeing 13.5-14.5 mpg in town and 16.8-17.5 hwy. The gas mileage (before and after the pedal commander) is very much related to how heavy-footed you are....much more so than my previous '17 Silverado w/ 6.2L engine.

 

[neeginan]

I picked up a 2020 Ram 1500 Big Horn Crew Cab 4x4 with the 5.7 eTorque and the 3.92. Drove from Atlanta to South Florida and got 19.8 mpg highway driving at between 75 and 80 MPH. Way better than what I expected. Hell, my wifes 2012 Nissan Pathfinder 4x4 with a 4.0L V6 only gets about 17 MPH under the same conditions. I expect the number will drop a bit when I put larger AT tires on, but still. I don't buy trucks for their fuel economy, but color me impressed by this one's so far.

First time out on the highway for any meaningful distance since I had the mild level/lift and AT tires installed. Bilstein 5100s front and back. Front set at #5 (1.7 in lift). Rear 1 inch motofab spacer. 275/65r20 AT tires on stock 20 inch wheels (34 inch tire vs 32 inch stock). So in the front she is 2.7 in higher than stock and 2 inches higher than stock in the back.

Highway mileage dropped from 19.8 to 17.7 mpg (taking account the 6.5% difference in mpg computer reported vs actual - 16.6 mpg was reported by the vehicle computer) . I was driving at ~75 mph (again taking into account the speedo error due to the 34 inch tires).

So the drop in highway mpg that I experienced was 2.1 mpg. I am not surprised. I raised the front 2.7 inches. The way FCA had the front of these leave the factory was really genius from a highway mpg perspective. With the fixed front air dam spoiler already low, plus the additional active air dam that deploys above 35 mph, they have really minimized air flow under the vehicle. If you look under our trucks (or any pickup for that matter), there is a lot of stuff for air to get caught up in underneath these vehicles, create a ton of drag. By lifting the front end almost 3 inches, I have introduced a lot of additional drag.

Of course I knew this was going to happen when I lifted it. I was expecting to lose between 1 and 2 mpg. I only drove about 140 miles so the initial estimate of a drop of 2.1 mpg may change a bit, but I expect it to be pretty accurate.

I still consider the highway mpg to be great for a mildly leveled/lifted 1/2 ton truck. Aerodynamic drag increases with the square of speed. So if I want to improve my highway mpg I will need to be willing to drive slower, maybe 65 mph instead of 75 mph - doubt that's going to happen. But if I did, I should see a significant improvement in fuel efficiency. By dropping from 75 mph to 65 mph, you are reducing the force required to overcome drag by 24.9%.

And there is probably a sweet spot on these vehicles, max speed where the truck will still utilize eco mode on the highway to improve mpg. I wonder what that is for a stock truck, and now for my truck. I noticed that my truck after the level/lift would never go into eco mode over 65 mph on the highway (unless descending a slight grade). I didn't think to check that before I raised it. I am going to try to figure that out on one of these trips for my truck now the way it is setup.

Edit: Inspired by being referred to as a nerd by EEnginerd, I decided to update my post with the other two variables that differed from my original MPG observation. 1. I now have a Tonneau cover on the bed. FYI, objective MPG tests of Tonneau covers on pickup truck beds show no difference in MPG with or without Tonneau cover. 2. I now have running boards. This probably adds a little bit of additional drag but I did install them tucked in behind the front wheels.

 

[EEnginerd]

First time out on the highway for any meaningful distance since I had the mild level/lift and AT tires installed. Bilstein 5100s front and back. Front set at #5 (1.7 in lift). Rear 1 inch motofab spacer. 275/65r20 AT tires on stock 20 inch wheels (34 inch tire vs 32 inch stock). So in the front she is 2.7 in higher than stock and 2 inches higher than stock in the back.

Highway mileage dropped from 19.8 to 17.7 mpg (taking account the 6.5% difference in mpg computer reported vs actual - 16.6 mpg was reported by the vehicle computer) . I was driving at ~75 mph (again taking into account the speedo error due to the 34 inch tires).

So the drop in highway mpg that I experienced was 2.1 mpg. I am not surprised. I raised the front 2.7 inches. The way FCA had the front of these leave the factory was really genius from a highway mpg perspective. With the fixed front air dam spoiler already low, plus the additional active air dam that deploys above 35 mph, they have really minimized air flow under the vehicle. If you look under our trucks (or any pickup for that matter), there is a lot of stuff for air to get caught up in underneath these vehicles, create a ton of drag. By lifting the front end almost 3 inches, I have introduced a lot of additional drag.

Of course I knew this was going to happen when I lifted it. I was expecting to lose between 1 and 2 mpg. I only drove about 140 miles so the initial estimate of a drop of 2.1 mpg may change a bit, but I expect it to be pretty accurate.

I still consider the highway mpg to be great for a mildly leveled/lifted 1/2 ton truck. Aerodynamic drag increases with the square of speed. So if I want to improve my highway mpg I will need to be willing to drive slower, maybe 65 mph instead of 75 mph - doubt that's going to happen. But if I did, I should see a significant improvement in fuel efficiency. By dropping from 75 mph to 65 mph, you are reducing the force required to overcome drag by 24.9%.

And there is probably a sweet spot on these vehicles, max speed where the truck will still utilize eco mode on the highway to improve mpg. I wonder what that is for a stock truck, and now for my truck. I noticed that my truck after the level/lift would never go into eco mode over 65 mph on the highway (unless descending a slight grade). I didn't think to check that before I raised it. I am going to try to figure that out on one of these trips for my truck now the way it is setup.

As a fellow nerd, I love to see the reasoning and numbers behind changes in performance. When mine comes in, I wanted to put the biggest A/T tires that would fit on the stock suspension on her, but you have definitely provided reasoning not to. Lol. I'll probably do it anyways, but thanks for posting this info.

 

[neeginan]

As a fellow nerd, I love to see the reasoning and numbers behind changes in performance. When mine comes in, I wanted to put the biggest A/T tires that would fit on the stock suspension on her, but you have definitely provided reasoning not to. Lol. I'll probably do it anyways, but thanks for posting this info.

Lol. You have no idea. I am an Engineer by Education. So yes, a nerd at times.

Force required to overcome drag = 1/2 X air density x drag coefficient x reference area x speed squared. I researched and used the actual CdA of the 2020 Ram 1500, and the actual air density for my location and elevation to determine the difference in force between 65 and 75 mph.

 

[EEnginerd]

Lol. You have no idea. I am an Engineer by Education. So yes, a nerd at times.

Force required to overcome drag = 1/2 X air density x drag coefficient x reference area x speed squared. I researched and used the actual CdA of the 2020 Ram 1500, and the actual air density for my location and elevation to determine the difference in force between 65 and 75 mph.

Nice. I should do that for mine, but I'm lazy af. What school did you get that fancy degree from?