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

[carguykeith]

I have a 120 mile commute at 80+ mph so the 3.21s were a no brainer. I found 82 to be the magic number and can consistently get 28mpg. I was only getting 36 in the 2019 Honda Accord I had before doing the same drive. The fuel mileage of these trucks is amazing!

 

[theblet]

I have a 120 mile commute at 80+ mph so the 3.21s were a no brainer. I found 82 to be the magic number and can consistently get 28mpg. I was only getting 36 in the 2019 Honda Accord I had before doing the same drive. The fuel mileage of these trucks is amazing!

I don’t think anyone of us has seen 28mpg at 82mph. Is mds engaged at this point?

 

[tidefan1967]

Got to be an Ecodiesel. There's no way in hell anyone's getting 28 MPG even with the Pentastar(much less a Hemi) at 80+ MPH. I would assume that you 3.21 guys are turning 2000 RPM thereabouts at that speed? Its pretty much a fact that fuel economy with a V8 drops significantly at or above that engine speed.

 

[theblet]

I know mines does. I get 15 to 17mpg at 80mph. Mds will not come on at those speeds unless going down a bridge

 

[BowDown]

Mines just under 2000 rpm at 80 but it's nowhere close to 28 mpg. At that speed I'm with @theblet around 16-17.
The fastest I can go and stay in the 20's is 75 mph and that hover around 19-20 mpg.
70-73 seems to be my 22+ mpg sweet spot

 

[lkjk]

I have 3.92s and got 20mpgs on a 1600 mile round trip from Denver to AZ and back, with a RTT over the bed, a fair amount was close to 90 mph. Not hand calc'd, just the trip calculator.

Gotta say, mpgs are better than my Jeep GC, which had 32s but that was it (no lift), at least trip computer to trip computer

 

[KWKSLVR]

Great write up. I'm happy with my choice to order 3.21. I decided that since the vast majority of my driving doesn't involve towing, and what I will tow will be around 4500lbs max including fuel/passengers/cargo, it just didn't make sense to go 3.92. In the even I decide to buy a small tractor one day, well, I guess I'll cross that bridge then.

 

[theblet]

agreed. happy with my choice too

 

[JF19Longhorn]

I have 3.92s and got 20mpgs on a 1600 mile round trip from Denver to AZ and back, with a RTT over the bed, a fair amount was close to 90 mph. Not hand calc'd, just the trip calculator.

Gotta say, mpgs are better than my Jeep GC, which had 32s but that was it (no lift), at least trip computer to trip computer

...how is that possible?

 

[CogitoErgoLevo]

I got 22 mpg with my truck doing 3000mile trip from LA to Boston.
I took I15 -> I70 -> I80 -> I90 and followed the speed limit. so stretches were at 80mph some at 75mph, some at 65.....
Most of the time I had Adaptive Cruise Control on and was not going for saving gas. I used 89octane the whole trip.
Every time I stopped I topped off the tank, so I mostly drove with a full tank.
3.21 ratio would give me even better mpg, but I am very satisfied with my current setup

I attached a picture of my lifetime mpg which happened to match my mpg for the trip.

 

[KWKSLVR]

...how is that possible?

2 things come to mind.

1) mileage calculators aren't perfect

2) how you break an engine in matters. I had a 1997 Trans Am with a full exhaust that got 34mpg in 6th gear on the interstate....

Crazy things happen sometimes

 

[IvoryHemi]

2 things come to mind.

1) mileage calculators aren't perfect

2) how you break an engine in matters. I had a 1997 Trans Am with a full exhaust that got 34mpg in 6th gear on the interstate....

Crazy things happen sometimes

T-56 had a very tall 0.50:1 sixth gear, along with the 3.42 axle that basically let the 5.7 loaf along at low rpm

 

[silver billet]

I have 3.92s and got 20mpgs on a 1600 mile round trip from Denver to AZ and back, with a RTT over the bed, a fair amount was close to 90 mph. Not hand calc'd, just the trip calculator.

Gotta say, mpgs are better than my Jeep GC, which had 32s but that was it (no lift), at least trip computer to trip computer

There is no way you're getting 20 mpg, with a 3.92, at 90 mph. That is completely impossible. Guys struggle to do 20+ with a 3.92 at 65 mph, at 90 I'd be surprised if you got better than 15 - 17.

 

[theblet]

For anything above 70mph, you should be in the teens for mpg unless you're going downhill.

 

[KWKSLVR]

T-56 had a very tall 0.50:1 sixth gear, along with the 3.42 axle that basically let the 5.7 loaf along at low rpm

Yep, and the headers and exhaust gave me just enough bump in low end torque to not need to downshift. I miss that car

 

[aromn]

3.92 ratio on a 2019 1500 longhorn. Just got back from a 4300 mile road trip from MN to the west coast and back (through the Rockies twice) and trip showed 19.6 mpg average. Wasn't hailing anything except 4 passengers

 

[392DCGC]

I got 22 mpg with my truck doing 3000mile trip from LA to Boston.
I took I15 -> I70 -> I80 -> I90 and followed the speed limit. so stretches were at 80mph some at 75mph, some at 65.....
Most of the time I had Adaptive Cruise Control on and was not going for saving gas. I used 89octane the whole trip.
Every time I stopped I topped off the tank, so I mostly drove with a full tank.
3.21 ratio would give me even better mpg, but I am very satisfied with my current setup

I attached a picture of my lifetime mpg which happened to match my mpg for the trip.

Sorry to burst your bubble, but that is NOT your lifetime MPG. The onboard computer has no way of tracking lifetime MPG. The trip computer MPG is a rolling average of the last 500~ miles, regardless of how many miles you have on the current trip. The ONLY way to track your MPG is to manually do it with an app like Fuelly. I know this sounds ridiculous, but if you don't believe me, check the regular MPG screen after driving a full tank of fuel through it without resetting... guarantee it will match your trip MPG screen even with 8K+ miles on the trip.

 

[Jrogowski83]

I never use ECO anymore; I don't like the sound and it doesn't do much in my usage. But my mistake, RAM engineers must have given us the 3.21 for no reason other than to laugh while we have fun discussing it.

See below for a 3 hour trip I did. This was still when I was using ECO, I've done the same trip later on with it disabled and got 9.8. When you can post a better MPG (converted, 9.7 L/100Km = 24 mpg) I'll rethink my viewpoint, until then, the 3.21 is the MPG king. I'm not saying you picked the wrong gear for your usage, or that you made a mistake. I'm saying it's a FACT that the 3.21 gets better MPG. You should re-read your second last post in this thread.

View attachment 49213

I'm glad I got the 3.21, I average mid 19s/mpg. I had a 4th gen with the 3.92 and rarely if ever saw over 16mpg

 

[lkjk]

There is no way you're getting 20 mpg, with a 3.92, at 90 mph. That is completely impossible. Guys struggle to do 20+ with a 3.92 at 65 mph, at 90 I'd be surprised if you got better than 15 - 17.

A fair amount at 90, not all of it, or even most, but enough to make a difference. Again, on the trip computer. Whether or not that's close to hand calc'd, who knows.

I do understand gear ratios so I'm not going to defend 3.92s and mpg, but i will say for a round trip like that, it depends on headwinds/tailwinds, drafting semis, elevation (in the mtns it makes a massive difference if you baby it up and go fast down) and for how much of that time as a % of total. To say it's impossible is false. I would also note there was virtually zero city driving outside of getting off the highway.

My trip comp usually says 15ish when i'm mostly around town with some highway, so i'm not saying i average anything out of the ordinary.

 

[Klicht87]

My average for highway driving involving hills and very minimal straightaways is about 19-20mpgs with 3.92s. Short in-town stop and go is around 16-17mpgs. I should mention those highway speeds are around 70 on a 65mph highway. RPMs are a hair under 2k.