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MPG effect of 3.92 rear end

Are you able to switch from 3:21 to 3:92? If so, will it be an expensive swap?
 
With the 8 speed transmission, 1st gear has plenty of grunt with either axle ratio. I have the 3.92 and honestly should have ordered the 3.21 with my highway use. Look at all the threads on lousey mpg and you'll see that more are running the 3.92.

I got drawn into the 3.92 hype from reading these threads. Yes, the truck is quicker...but 2300 rpm on the interstate at 14-15 mpg isn't worth it for me. The truck is still slow relative to real performance vehicles. I'm not racing anyone and I don't tow over 8k.
 
My truck is a Bighorn 4x4 with the Off Road Package and 3:92 gears. I have the front lifted 2.5 inches and am running 35-12.50-18 Nitto Ridge Grappler tires. I got the 3:92's because I knew I wanted 35's and I will do some towing up to about 5500 pounds.

I know my truck is not broke in yet, but even with the factory wheels and tires the best I have ever seen on the instant fuel economy has been 18-19, and that is driving 50-55. At 70, it was showing instant mileage of 14-16. I filled up the other day and drove mostly two lane highway and a good bit on the interstate and the tank was averaging under 15. Now, with the larger tires my speedometer/odometer is off 8 percent, so I have to add that to my miles when I hand calculate the mileage.

Today, I had to drive to the airport in St. Louis to pick up a friend. Since gas is so much cheaper down there i went ahead and filled up even though I was only down to about 3/4 of a tank. The computer showed I was getting 14.8 MPG. Hand calculated it came out to 17.9. That is a HUGE difference, and yes I calculated it correctly.

I was really disappointed when watching my computer mileage, especially with how easy I was driving. Yea, I know the fuel is winter blend which has a big effect on mileage, and I know about fluids and operating temperatures, and the bigger tires, and..........but i was still a little disappointed. But, the fact that the computer was that far off, and now knowing it is much better than I thought, I am actually pretty happy right now.

Could it be better? Sure. But a full size, hemi powered truck, on 35's with 3:92's, I am actually pretty stoked with 17.9 MPG. Will it get better? I am sure it will.

Sorry about the rambling, but I am fairly pleased with my fuel economy all things considered.
 
Ridge grapplers are heave tires. 12ply I believe. 17.9 is pretty damn good. I ran ridge grappler on my ecodiesel and got 21-22 average.
 
ExcursionDiesel said:
With the 8 speed transmission, 1st gear has plenty of grunt with either axle ratio. I have the 3.92 and honestly should have ordered the 3.21 with my highway use. Look at all the threads on lousey mpg and you'll see that more are running the 3.92.

I got drawn into the 3.92 hype from reading these threads. Yes, the truck is quicker...but 2300 rpm on the interstate at 14-15 mpg isn't worth it for me. The truck is still slow relative to real performance vehicles. I'm not racing anyone and I don't tow over 8k.
Click to expand...

Obviously higher annual mileage will affect these numbers but if we were to pretend you got 19MPG with 3.21 you may save 30-40 per month in fuel. I also got drawn in but I am glad I decided on 3.92 because I ended up with bigger tires and more people try to move to 3.92 from 3.21 then the other way around.
 
Looking at fueleconomy.gov (EPA website) I see that there are only four v8 models rated: 5.7 4x2, 5.7 4x4, 5.7 eTorque 4x2, and 5.7 eTorque 4x4. That’s it. No separate ratings for extended cab vs crew cab. No separate rating for 3.21 vs 3.92. No separate rating for trucks with higher ride height or off-road tires. All these options are likely to negatively affect mpg. I assume that they used test mules that were lightly optioned, lowest trim and were THE most efficient configuration, namely extended cab, 3.21, Tradesman and no off road package.

Then I look at Fuelly.com where I see there appears to be .6 to 1 mpg difference between extended cab vs crew cabs when selecting the 5.7 Gas engine. To me that one is confirmed.

So assuming you lose .6 to 1mpg for a crew cab and you lose 1 to 2mpg for the 3.92, maybe another .5 mpg for off-road ride height/tires and another .2mpg for a heavily optioned/higher trim truck.....you see where this is going. Just guessing on most of these numbers but there is likely some negative effect for each.

Yeh, maybe FCA should have rated some of these separately to help set expectations for their customers even if not required by the EPA.
 
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ExcursionDiesel said:
With the 8 speed transmission, 1st gear has plenty of grunt with either axle ratio. I have the 3.92 and honestly should have ordered the 3.21 with my highway use. Look at all the threads on lousey mpg and you'll see that more are running the 3.92.

I got drawn into the 3.92 hype from reading these threads. Yes, the truck is quicker...but 2300 rpm on the interstate at 14-15 mpg isn't worth it for me. The truck is still slow relative to real performance vehicles. I'm not racing anyone and I don't tow over 8k.
Click to expand...

70mph is 1900 rpm.

People seem to think highway means 80-85.
No, highway is 55-65.

But too each their own. I can drive 68 and get 21 with 3.92s in mid Missouri highways. Up and down hills.
 
768mph said:
70mph is 1900 rpm.

People seem to think highway means 80-85.
No, highway is 55-65.

But too each their own. I can drive 68 and get 21 with 3.92s in mid Missouri highways. Up and down hills.
Click to expand...
In TN we have 4 lane divided highways posted at 65 and 70 mph. I run 10 over routinely. My truck is as heavy as they get and has the ORG. I get 21 mpg at 65 on flat highway. By 70, I'm down to 18-19 mpg. 80 mph drops to 14-15 mpg with ~2250 RPM. Peak economy is around 46 mph with MDS on and flat ground the dash shows 28 mpg. Any faster and it drops.
 
ExcursionDiesel said:
In TN we have 4 lane divided highways posted at 65 and 70 mph. I run 10 over routinely. My truck is as heavy as they get and has the ORG. I get 21 mpg at 65 on flat highway. By 70, I'm down to 18-19 mpg. 80 mph drops to 14-15 mpg with ~2250 RPM. Peak economy is around 46 mph with MDS on and flat ground the dash shows 28 mpg. Any faster and it drops.
Click to expand...

My truck is similar to yours... 70mph up to WI and back to FL average was 15mpg. Worst was 13, Best was 16... all based on fuelly.
 
ExcursionDiesel said:
In TN we have 4 lane divided highways posted at 65 and 70 mph. I run 10 over routinely. My truck is as heavy as they get and has the ORG. I get 21 mpg at 65 on flat highway. By 70, I'm down to 18-19 mpg. 80 mph drops to 14-15 mpg with ~2250 RPM. Peak economy is around 46 mph with MDS on and flat ground the dash shows 28 mpg. Any faster and it drops.
Click to expand...

Yes sir. You’re correct. I’m always on or around a military base, so speed limits are always 20-40mph for travel. Im Active Army and 3.92 was better choice due to the about of “city” driving I’ll have to do. I’m about 17.5 with 90% of stop and go with low speed limits. Highway I can do 65-68 and get 21.
My previous 18 Charger 392, at 73 I was able to get 27 but regularly 25 on hwy.

dblankenbaker said:
Here's why...
http://ontheworldmap.com/usa/usa-speed-limits-map.jpg
Click to expand...

Yes, I am aware of the speed LIMITS. Key word is LIMIT, not the speed required to travel. Limit is the most you’re legally allowed to travel on that particular road if posted in white. Yellow speed limit signs are mere suggested speed limits (I.E. curves or freeway exits)
Most highways have a 20-30mph window.
For example, i-44 in Missouri is 70mph limit but a 40mph minimum.

I understand your argument. But don’t confuse speed limit with speed requirement. (Sincere)
 
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I watched a video interview with an automotive engineer on this topic. His thoughts were there would be very little difference in the city and maybe a 1 MPG hit at highway speeds with higher ratio rears.

Just me talking here now (so take it for what it is worth), I suspect the vehicle could stay in cylinder deactivation longer with an higher ratio rear. Helping to offset some of the loos due to a 3.92. I have noticed mine will stay in "eco" on slight grades and even very mild acceleration.
 
My two cents on the 3.92 over the 3.21 in very few words if it helps. 3.21 pulls at top limit around 8000 lbs. the 3.92 will do 11000 Lbs. If you pull you anything you must stop as well as take off. The 3.92 helps to stop and manage any weight with push back the 3.21 will have significantly less. The MPG empty bed no pulling is only about 1 to 2 mpg difference. Why not have the extra pull capacity and better weight management for the same or near the same price of ownership and fuel costs?(:
 
Zeronet said:
If I were planning to tow 8000+ for any significant distance or with any frequency, I’d buy a 2500. Just my comfort level.
Click to expand...

The 1500 with 3.92 both 2014 and 2019 models tow 8000 lbs very easy and safe with no issues, with of plenty of head room left over in max tow caps. of 10200 2014 and 11300 on 2019. If I was to tow say 10000lbs plus at the top of the weight caps of the 3.92 I too would move to a 2500 with 4.10 gears and half ton more in over all weight of truck. The 3.92 helps to push back very well and manage the weight keeping many from spending more money on a hd 2500 plus at significantly higher cost of ownership and insurance when not needed. But all to their own in feeling of Comfort.
 
I have a 5.7 3.92 with etorque and bf Goodrich ko2 stock size... just finished a 2k mile trip... mostly freeway and highway miles... 15.5mpg is my average.. I have 2900miles total. seems so low for a truck that is advertised as 22highway/17city 19 combined. I’m disappointed... my 2010 ram bighorn 1500 5.7l did better...
 
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