MPG effect of 3.92 rear end

[dutchman187]

OK. I have about 1300 mile on my truck so I know it is not fully broke in. I just filled after a little over 250 miles. 80 percent was driving interstate (65 and 70 MPH). I drove the speed limit There was a headwind going south and a good tailwind going north. I live in the country, so most of the other driving is 55 MPH going to and from town, with a little in town driving. I really babied it this tank (really, I baby it every tank). I have also been filling with Shell, although only 87 octane.

When I filled this morning I averaged 13.37. The computer showed 13.4. After I filled, I checked the computer instant mileage at different speeds (I have done this many times). Here are my best observed MPG at stated speeds:

45 MPH - 18 MPG
50 MPH - 18 MPG
55 MPH - 16 MPG

Driving 70 on the interstate last trip, it would bump 15 going on a slight downhill, but showed 13 the majority of the time. This was with a tailwind.

While I know many are tired of hearing MPG complaints, I am really wondering if others have experienced mileage this far off EPA estimates. I know Ram should not have bunched all of the Hemi trucks in the in the same category. I know mine has 3.92's and the Off Road Group. I know mine has 35 inch tires now, but it really wasn't much, if any better with the factory tires.

I saw an old post where there were two people with Rebels posting to Fuelly and they were averaging less than 13. I guess mine is equipped like a Rebel so I should expect no difference, but I really expected to get 17-18 running the speed limit on the interstate, not 13.

Have any others experienced the same poor mileage driving easy? More importantly, was there a mechanical issue causing poor mileage?

Love the truck. The fuel economy......not so much.

Last weekend I was able to drive some two lane country hey. On completely flat road at 60, my instant was steady at 21. At 62 mph, it dropped to 18. However, at 70 it was 17-18. Above 72, it dropped more to around 15-16.

 

[Troop2865]

I just did a 350 mile trip today. On the way out (175 miles) I did speed limit +5 (75) into a 15 MPH head wind and averaged 17.0 MPG. On the way back, I did speed limit +2 (72) with no wind and averaged 19.6 MPG. I can live with that considering I still get the benefit of 3.92 gears.

Thanks for the replies guys. As I said, I still love the truck.

I am running the tires at 40 psi. Had them at 44, but the middle of the tire was taking most of the wear.

Billy James _ I want your fuel economy.

 

[Billy James]

Thanks for the replies guys. As I said, I still love the truck.

I am running the tires at 40 psi. Had them at 44, but the middle of the tire was taking most of the wear.

Billy James _ I want your fuel economy.

I think the key is keeping it below 2000 RPM. When my truck is below 2000 RPM the ECO light is on quite a bit. That is why I did 72 MPH on the way home; it sat at around 1800-1900 RPM. Of course, I do live in Florida so I don't have a lot of hills to deal with. I assume that if it was not so flat my fuel economy would be worse. I am also running Shell 89 octane.

 

[Horkn]

Did you test that ORP 3.21 truck?

Not yet. I plan to fairly soon

 

[glennf14]

Hey guys, I have a 4x4 Limited with mostly every option, including the 3.92 gears (I’ll update my signature to be specific).

Only 450 miles on the truck so far. My commute is ~25 miles - mosty flat with some gentle hills. I live in central NC. I’d estimate it’s 70% highway (50 to 75 mph for me) and 30% city (0 to 50 mph for me). Certainly not LA traffic but 5pm can still be kinda rough (love the adaptive cruise).

So far I’m averaging 16.5 mpg on my commutes. It’s kinda funny how consistent it is. I’m hoping as the truck gets more miles it’ll creep up to 18. I usually reset the trip B every day and take a new measurement. I haven’t hand calculated anything though. Does anyone recommend an app for the “hand calculation”?

I whole heartedly agree with the folks here saying that anything over 2k rpm or ~65mph causes a quick drop off. Eco light comes up much more often between 50 and 65mph.

Also don’t forget to enable the Eco light in your Uconnect settings if you’re not seeing it!!

 

[GRAYWOLF]

I use fuelly app to track fill-ups. I only got 11.7 on my first tank.

Sent from my SM-G965U using Tapatalk

 

[Billy James]

I use fuelly app to track fill-ups. I only got 11.7 on my first tank.

Sent from my SM-G965U using Tapatalk

My first tank was 12.5 MPG. I'm at about 4,000 miles now and I'm seeing and average of about 16.8 in mixed driving. It should get better; but it will never be great. I think that considering we have a truck with a V8, 4x4, and 3.92 gears that weighs close to 6,000 pounds; anything above 15 is not too bad. My old 2003 RAM 1500 ST Regular Cab, 4x4, 4.7 averaged 10 MPG and on a good day might have gotten 12 MPG. That is why my other car is a 2018 Chevrolet Malibu 1.5T.......36 to 41 MPG all day everyday.

 

[Gondul]

So far I’m averaging 16.5 mpg on my commutes. It’s kinda funny how consistent it is. I’m hoping as the truck gets more miles it’ll creep up to 18. I usually reset the trip B every day and take a new measurement. I haven’t hand calculated anything though. Does anyone recommend an app for the “hand calculation”?

Fuelly app for sure... very simple to use and maintain.

I think the key is keeping it below 2000 RPM. When my truck is below 2000 RPM the ECO light is on quite a bit. That is why I did 72 MPH on the way home; it sat at around 1800-1900 RPM. Of course, I do live in Florida so I don't have a lot of hills to deal with. I assume that if it was not so flat my fuel economy would be worse. I am also running Shell 89 octane.

Yep... keeping it below 2K rpm is about right... my ECO does not really come on at all over 55/60.
A down and dirty calculation...

70mph is about 2017rpm
65mph is about 1873rpm
60mph is about 1729rpm
55mph is about 1585rpm

 

[Electrical]

...

Does anyone recommend an app for the “hand calculation”?

...

aCar (Car Management, Mileage) by Fuelly, LLC

 

[NDanecker]

IMO, the explanation for very similar mileage between the 3.92 and 3.21 is the ability of MDS to stay active more with 3.92 vs. 3.21. Remember - 3.92 ratio will give the 4 cylinder mode a mechanical advantage and less likely kick out when the truck requires more torque to maintain speed uphills, etc. thus staying in 4 cyl mode yielding better mileage.

 

[Gondul]

IMO, the explanation for very similar mileage between the 3.92 and 3.21 is the ability of MDS to stay active more with 3.92 vs. 3.21. Remember - 3.92 ratio will give the 4 cylinder mode a mechanical advantage and less likely kick out when the truck requires more torque to maintain speed uphills, etc. thus staying in 4 cyl mode yielding better mileage.

Yeah... mine is rarely in ECO/MDS unless I'm under 50mph or going down hill....

 

[GRAYWOLF]

I use fuelly app to track fill-ups. I only got 11.7 on my first tank.

Sent from my SM-G965U using Tapatalk

11.7 on my second tank (computer said 12.3)
Halfway through third tank and computer is showing 13.2

 

[El-jefes-silver-laramie]

I do 99.9% city driving and I have the 3.92 on my 2019 Ram. My MPG is approx 13.5mpg right now but I am only at 220 miles so the engine isn't even broken in yet.

I just turned over 1700 miles in my 2019 4x4 Crewcab with 3.92 I’d getting 13.6 in town and 17 on the interstate.

 

[2wd]

Still breaking in of course (only 1k miles), but I've got a 3.21, crew cab, non etorque 4x4. Getting 14-15 in town (very short 3-5 mile trips, engine can't warm up half the time), and 20.5 on the highway on a ~350 mile round trip long drive. After seeing some of the numbers posted I feel lucky.

I would imagine the only mpg gains from 3.21 would be at highway. In around town driving, the 3.92's will be in hypothetical 7th while the 3.21 in 6th essentially running similar MPH and RPM.

 

[Joecameron69]

aCar (Car Management, Mileage) by Fuelly, LLC

Im using the website

www.fueleconomy.gov/mpg

Lets you compare to other drivers of the same truck, but going to look up fuelly, thanks.

 

[EGD]

Still breaking in so this pic may not be indicative number. I like to hand calculate. Driving yesterday on our flat highways.

 

[Willwork4truck]

Wish I would have read this before I ordered the 3.21. Do mostly highway driving, not towing and went that route for the MPG.....sounds like it's a wash either way only better acceleration on the 3.92....oh well

A little late to this party and its a perennial head scratcher for many (why did FCA drop the 3.55 anyway?) but I was going to buy a 3.92 even though this is a highway trip machine and grocery getter.

I never tow and there are no mountains nearby. I still would have preferred the 3.92 as its better to have the power than not, and could be better for resale, yet the truck I bought off the lot was a 3.21. Oh well, wont really matter for my uses. Now if you race, tow or live near the Rockies, 3.92’s.

My 2015 F150 with the 2.7 was faster and never dropped a gear on good sized hills but it also did not live up to its mpg claims.

 

[Neurobit]

Ignorance if bliss I guess. I feel my 3.21 is more than ample power. Since I have not driven a 3.92, I have nothing to compare it to.

Cheers,

 

[ABQ Mike]

Thank you both for your input, and I'm sure I'll be following your recommendation! If there really isn't a difference in MPG, then why is FCA offering the 3.21 at all? The logical side of me says that they had to have a reason to offer the 3.21...but what is that reason?

I'm so glad I found this forum, as I've learned a lot...I'm very thankful I didn't already order one, as this forum has modified some of the choices I would have made 2 months ago!

Couple ways to look at this, as driving conditions and style will affect the outcome either way.

On a completely flat surface at a steady speed, say 75 mph, comparing two identical trucks, with the only difference being gear ratio, the lower gear will get better fuel economy as the engine is turning a slower speed. This is a general reference not factoring any extenuating circumstances.

However... insert variables into play and things change very rapidly. The following are a few scenarios as such.

Around town driving mostly - this removes gear ratio from the equation for multiple reasons. First off, higher ratios require less effort and pedal force to accelerate. 30% throttle may give you the acceleration you desire with a higher gear, where as you may need 50% to achieve the same with a lower gear ratio. Second, you generally never sit in the overdrive gears long enough to notice the economy difference in town as you don’t achieve speeds requiring such gears. Third, when factoring in hills and terrain other then flat, the engine must downshift to the appropriate gear to maintain speed where as with higher gear ratios, often it will leave the truck in overdrive or higher gears.

Towing - should be self explanatory, but with a heavier load behind the truck, the lower gear ratio will require downshifting and more pedal force to maintain speeds or accelerate, where as higher gear ratios do not, and often can maintain speeds even up hill in overdrive.

Higher speed cruise, hills, mountains, wind resistance, lift kits, large tires etc... same deal here, once you reach a point where the throttle has to be opened substantially to maintain a speed, or accelerate to your liking, the higher gear ratios are going to have an advantage as it will require less effort to maintain, even though it may be at a higher rpm.

Low gears are great if you drive across country, or on long distances often, or drive responsibly, but for any other conditions, I feel higher ratios are always advantageous.

For what it’s worth, I bought my truck in Tucson and drove it back to Albuquerque through flagstaff (stopped in Phoenix) and maintained 85-90 mph the entire way, including through the mountains, and still averaged about 17-18 mpg for the trip, with the 3.92 gears. I left it on cruise and didn’t have to touch it through the mountains unless I got stuck behind trucks, and I imagine had I had the lower ratios, I likely would have been hot rodding it much more to get the speed and acceleration I needed, worsening the fuel mileage.

 

[Ramit392]

Having a 3.92 is like having more HP. The hp has cost to keep it on board and it nice when you want it! The lower hp has no additional costs and when you need it its not there,but having nothing to compare it with,one may never know what they missed No matter which gear you have you still have a great looking truck and can enjoy it no matter what