Some Real World MPG Numbers

[BowDown]

MPG is directly related to speed + rpms. Both trucks have the ability to use all 8 gear ratios in the city, meaning the rpms will drop as low as possible for the speed you're at. That means that both trucks can drive around in the city at 1200 - 1500 rpms, no difference.

On the highway, the 3.92 runs out of gears whereas the 3.21 can upshift one more time, dropping the rpms yet again. On the highway when both are in 8th, the 3.21 will always be 400-ish rpms lower, that's where the difference is.

3.21 geared truck will do it with 20% less rpm in every gear though.
3:21 geared trucks consistently get better fuel mileage vs the 3:92 truck as posted by owners here, I really don't get the debate

 

[silver billet]

3.21 geared truck will do it with 20% less rpm in every gear though.
3:21 geared trucks consistently get better fuel mileage vs the 3:92 truck as posted by owners here, I really don't get the debate

Yes, but you have to look at the gear ratios available for each truck; the math works out so nicely for the 3.21 and 3.92 such that once you're moving, both trucks are using the same gear ratios (giving pretty much identical rpms for a given speed). Doesn't matter that the one truck is in 3rd and the other in 4th when both gear ratios in that example are equivalent.

So while the trucks may be in a different numerical gear, the end up using the same gear ratio and same rpms and same speed.

This holds true specifically for 3.21 vs 3.92. If it was 3.21 vs 4.10 or whatever the math won't work out that nicely, but still. It's possible to find a given MPH such that the 3.92 is revving lower (just after an upshift) whereas the 3.21 might be hanging out near the top of the gear in a higher rpms.

 

[Hydroblueguy]

First highway trip with my 2021 5.7 etorque, 3:21. Truck now has 650 miles, so not even broke in. I never drove over 60 and when I got home the trip mpg is at 22.7! I’d guess it’s closer to 21.5 but I think that good, all factory, not pulling anything! 87 cheap gas!

 

[theblet]

I went on a run with the cruise set at 70 today. Mds did not kick in at all. I was making 19 to 21 mpg.

 

[The_Dude]

I think tires also have a lot to do with it. I put Michelin's in and it helped 1-2%.

 

[BowDown]

Yes, but you have to look at the gear ratios available for each truck; the math works out so nicely for the 3.21 and 3.92 such that once you're moving, both trucks are using the same gear ratios (giving pretty much identical rpms for a given speed). Doesn't matter that the one truck is in 3rd and the other in 4th when both gear ratios in that example are equivalent.

So while the trucks may be in a different numerical gear, the end up using the same gear ratio and same rpms and same speed.

This holds true specifically for 3.21 vs 3.92. If it was 3.21 vs 4.10 or whatever the math won't work out that nicely, but still. It's possible to find a given MPH such that the 3.92 is revving lower (just after an upshift) whereas the 3.21 might be hanging out near the top of the gear in a higher rpms.

8th is roughly 5-600 rpm less in a 3:21 reuck vs a 3:92, that's not the same.
I drive in Dallas, mainly city and even the highway driving is rush hour driving and I'm averaging high 17-18's every tank; last tank averaged 17.6 for 413 miles on 23 gallons. They are not the same

 

[BowDown]

First highway trip with my 2021 5.7 etorque, 3:21. Truck now has 650 miles, so not even broke in. I never drove over 60 and when I got home the trip mpg is at 22.7! I’d guess it’s closer to 21.5 but I think that good, all factory, not pulling anything! 87 cheap gas!

71-72 mph seems to be the sweet spot for the 3:21 gear. I can get 22-23 at that speed, 24 if its stays flat for a while . 75 mph drops to 21 mpg and 80 mph down to 20 mpg

 

[IvoryHemi]

8th is roughly 5-600 rpm less in a 3:21 reuck vs a 3:92, that's not the same.

You won’t see a 500 rpm difference until 100 mph

At 60 mph it’s ~300 rpm
At 65 mph it’s ~323 rpm
At 70 mph it’s ~350 rpm
At 75 mph it’s ~373 rpm
at 80 mph it’s ~400 rpm

 

[BowDown]

You won’t see a 500 rpm difference until 100 mph

At 60 mph it’s ~300 rpm
At 65 mph it’s ~323 rpm
At 70 mph it’s ~350 rpm
At 75 mph it’s ~373 rpm
at 80 mph it’s ~400 rpm

My old truck with 3:92's was at 1900-2000rpm at 67 mph. My 3:21 truck is at 14-1500 rpm at 70mph. Its math, there's a 20% final drive difference across every gear which amounts to a ~500-600 rpm

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

Plug in the 3:21 and 3:92 for each trans gear, you'll see about 5-600 rpm difference between the 3:21 and 3:92 in every gear
Here are the 8HP75 trans gears
1st-5.000:1
2nd-3.200:1
3rd-2.143:1
4th-1.720:1
5th-1.314:1
6th-1.000:1
7th-0.822:1
8th-0.640:1
R-3.456:1

 

[ColoradoCub]

You won’t see a 500 rpm difference until 100 mph

At 60 mph it’s ~300 rpm
At 65 mph it’s ~323 rpm
At 70 mph it’s ~350 rpm
At 75 mph it’s ~373 rpm
at 80 mph it’s ~400 rpm

And switching from the factory 32” tires to 34” tires drops the rpms in a 3.92 equipped truck by right at 200 rpms at 70.

 

[IvoryHemi]

My old truck with 3:92's was at 1900-2000rpm at 67 mph. My 3:21 truck is at 14-1500 rpm at 70mph. Its math, there's a 20% final drive difference across every gear which amounts to a ~500-600 rpm

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio

purperformance.com

Plug in the 3:21 and 3:92 for each trans gear, you'll see about 5-600 rpm difference between the 3:21 and 3:92 in every gear

my statement was correct.

Here are the rpm using your website and the 32.1” tire (285/45/22) size on 3.21 vs 3.92

60 mph: 1649 - 1350 = 299 rpm
65 mph: 1786 - 1463 = 323 rpm
70 mph: 1924 - 1575 = 349 rpm
75 mph: 2061 - 1688 = 373 rpm
80 mph: 2199 - 1800 = 399 rpm
100 mph: 2749 - 2251 = 498 rpm

 

[IvoryHemi]

Here are the 8HP75 trans gears
1st-5.000:1
2nd-3.200:1
3rd-2.143:1
4th-1.720:1
5th-1.314:1
6th-1.000:1
7th-0.822:1
8th-0.640:1
R-3.456:1

You’re using the wrong ratios.

Here are the correct 8HP75 transmission gear ratios for Ram:

 

[BowDown]

my statement was correct.

Here are the rpm using your website and the 32.1” tire (285/45/22) size on 3.21 vs 3.92

60 mph: 1649 - 1350 = 299 rpm
65 mph: 1786 - 1463 = 323 rpm
70 mph: 1924 - 1575 = 349 rpm
75 mph: 2061 - 1688 = 373 rpm
80 mph: 2199 - 1800 = 399 rpm
100 mph: 2749 - 2251 = 498 rpm

Except you missed this statement
"With our RPM calculator, you can calculate your engine's RPM based off of your vehicle's speed, tire diameter, rear gear ratio, and transmission gear ratio. Keep in mind that our calculator does NOT take into account drive train loss, road and weather conditions, or the skill of the driver. "

On a chassis dyno, a lower gear (numerically higher) always has more parasitic loss than a higher gear hence more rpm to maintain same speed or do you think 300 rpm is the difference between these trucks getting 3-5 mpg different on the highway?
Now, go drive at 70 mph and see where your RPM's are. At 70, mines 14-1500. Many 3:92 guys have already posted that they are ~1900rpm at 70 mph

 

[BowDown]

You’re using the wrong ratios.

Here are the correct 8HP75 transmission gear ratios for Ram:

View attachment 88201

You win, enjoy your 16 mpg and I'll keep slumming it at 18mpg in the city

 

[Mopar_Gummy]

Averaging about 12mpg. I drive mostly city. Not spirited driving, just putting around.
Truck is
5.7 with eTorque
3.92
Crew cab
4x4
Off-road 32"
Varies if i drive it on "normal" or "aero" air suspension

Have seen best average of 13.5 mpg, least of 11mpg. I always fuel 93 octane. Truck has about 25k miles

 

[19 North]

Is everyone running the recommended tire pressure on the door jam or the tire itself? I have the 275/65/18 wildpeak and at 38psi they look slack,so I bumped them to 43psi. The sidewall calls for 50psi for max load.
I run 90 octane non-ethanol and evidently they dropped the first of the summer gas last week. I’m up to 17.5mpg and hope to break 18 in a few weeks.

 

[IvoryHemi]

Except you missed this statement
"With our RPM calculator, you can calculate your engine's RPM based off of your vehicle's speed, tire diameter, rear gear ratio, and transmission gear ratio. Keep in mind that our calculator does NOT take into account drive train loss, road and weather conditions, or the skill of the driver. "

On a chassis dyno, a lower gear (numerically higher) always has more parasitic loss than a higher gear hence more rpm to maintain same speed or do you think 300 rpm is the difference between these trucks getting 3-5 mpg different on the highway?
Now, go drive at 70 mph and see where your RPM's are. At 70, mines 14-1500. Many 3:92 guys have already posted that they are ~1900rpm at 70 mph

You said:

“Its math, there's a 20% final drive difference across every gear which amounts to a ~500-600 rpm”

“Plug in the 3:21 and 3:92 for each trans gear, you'll see about 5-600 rpm difference between the 3:21 and 3:92 in every gear”

Yes it is simple math. 3.92/3.21 = 22% more gear. Divide the above rpm’s and you’ll find there is a 22% difference between the given rpm. Which is what you said, but your math was off. You will not find a 5-600 rpm difference until 100+ mph

Tire size will also pay a factor as said above.

Drivetrain loss on 3.21 vs 3.92 is negligible. That’s not why lower gears read lower on the dyno.

 

[IvoryHemi]

You win, enjoy your 16 mpg and I'll keep slumming it at 18mpg in the city

4x2 certainly helps

 

[Swicago]

Just did a 1500mi trip from Chicago IL, to Louisville KY, to Sweatwater TN, to Gatlinsburg TN, to Nashville TN, to St.Louis MO, back to Chicago IL.
Mixed terrain of of up and down hills.
Always 10 over speed limit, mostly was able to go 80 in a 70 zone(probably 90% of the time).
87 octane
3.92 gear crew cab laramie, with 4 adults.
17mpg ... could have probably got 20ish if I'd stay around 60mph, but no thanks...lol

 

[BowDown]

You said:

“Its math, there's a 20% final drive difference across every gear which amounts to a ~500-600 rpm”

“Plug in the 3:21 and 3:92 for each trans gear, you'll see about 5-600 rpm difference between the 3:21 and 3:92 in every gear”

Yes it is simple math. 3.92/3.21 = 22% more gear. Divide the above rpm’s and you’ll find there is a 22% difference between the given rpm. Which is what you said, but your math was off. You will not find a 5-600 rpm difference until 100+ mph

Tire size will also pay a factor as said above.

Drivetrain loss on 3.21 vs 3.92 is negligible. That’s not why lower gears read lower on the dyno.

I hear you, check your rpm at 70 mph, I'm ~1400