3.55 vs 3.92 rear end

[kmcdonough]

I am thinking of downsizing from my 2500 CTD to a 1500 with the eco-diesel. I know the 3.92 rear ups my towing some, but I am wondering how it will effect my fuel mileage as a daily driver. For those of you who have a new eco-diesel, can you tell what rear end you have and your fuel mileage. Thanks.
Kevin

 

[Patty]

3.55 currently averaging 22.1 US MPG on fuelly but...winter fuel, winter conditions, idling and warm-ups, plus I drive normal speeds with traffic not trying to set a mileage record. I will expect 26 - 27 MPG highway again come summer. My towing with this truck will be either a 2000 pound enclosed motorcycle trailer or a 2000 pound boat so I wasn't concerned with finding a truck with 3.92. I am however looking at a 4" lift with only slightly bigger than stock tires so I hope I don't regret it after that. I doubt it.

 

[HPSimpson]

I have 3.92 axle and tow 16' boat weights about 1900 lbs., towing on California freeway at 55 miles per hour average full economy is 22 to 24 miles per gallon strictly depends on weather conditions, windy or wet roads.

 

[S.Morris]

Patty
Is that US or Imp mileage? Lifting the truck will hurt your mileage, guaranteed, especially at highway speeds.

 

[Patty]

It's US MPG.

 

[Waldo Pepper]

Mine has 3.55 locking rear axel and duel factory exhaust. Winter stop and go is running 22-25mpg, level interstate at 71mph I average 28-30mpg depending on if winter or summer. Towed 1500lb trailer, truck bed loaded as was interior moving last summer with 3 trips averaged 28mpg thru Ozark Hills. I currently have 15,000 miles on the truck and have had no issues with truck, except Goodyear tires that are only going to last till about 20,000 miles. Wear pattern is perfect, but tread is border line pass for state inspection. I will buy new tires before October.

 

[kmcdonough]

thanks for the info. I think I am going to stick with the 3.55 that is std. with the diesel. I'm guessing that Chrysler felt this to be a good comprise for mpg vs. towing. I won't tow that often, but when I do it will be 6-7k. With regard to a lift and bigger tires, how does that effect the rear end and mileage? I'm guessing it will reduce the torque aspect because of the bigger tire circumference, but once you're rolling, won't you get better fuel mileage (longer distance per rotation)?
Kevin

 

[jhenderson]

A lift and bigger rubber raise the profile of the truck and create more wind resistance thus costing fuel economy. All of the mfgs spend much time and money trying to cheat the wind to gain a couple of percent in MPG.

 

[Roc Doc]

I got mine with 3.92's with the idea that I may go to an oversized tire. The revs at 75mpg are still close to 2k. Maybe a 2wd tradesman could get away with 3.55's but the extra weight of these loaded 4x4 trucks do better with the lower gears...IMHO

 

[Waldo Pepper]

I got mine with 3.92's with the idea that I may go to an oversized tire. The revs at 75mpg are still close to 2k. Maybe a 2wd tradesman could get away with 3.55's but the extra weight of these loaded 4x4 trucks do better with the lower gears...IMHO

The gearing in transmission negates the difference unless towing, just normal driving even with 3:55 gears my truck never uses 1st gear unless I select the gear. With a lot more torque at 1000 rpm then a big Hemi, moving this 6200lb truck is no strain with 3:55 gears and at 70mph on flat highways rev's are about 1800 IIRC and 30/32mpg is the norm...I also have a bed cover that removes the airbrake effect of tailgate.

The 3:92 I test drove over weekend at 70mph was pulling about 2100rpm IIRC and never was able to move the bar graph mpg gage above 24mpg, following weekend when I drove the truck I bought the same way and same roads all weekend it did 23mpg in town and 31mpg on highway.

A friend has one with about 35,000 miles on it and it has 3:92 gears with single exhaust and he gets 24mpg highway cruising 70mph and says his combined mpg average over that 35,000 plus miles has been 22.x and says his Hemi he traded in only got 13mpg highway and averaged about 10mpg with work trailer and had harder time getting his work trailer moving. He always commented over the years that his trucks would pass everything but a gas station, he's never said that about the Eco.

 

[Roc Doc]

The gearing in transmission negates the difference unless towing, just normal driving even with 3:55 gears my truck never uses 1st gear unless I select the gear. With a lot more torque at 1000 rpm then a big Hemi, moving this 6200lb truck is no strain with 3:55 gears and at 70mph on flat highways rev's are about 1800 IIRC and 30/32mpg is the norm...I also have a bed cover that removes the airbrake effect of tailgate.

The mfgs of modern trucks design the body to be as aerodynamically efficient as possible with the bed uncovered and the tailgate up. If you look at the top sill of the tailgate it looks like a spoiler, which it is. The bed covers don't hurt anything, but they don't really help with the aero effieciency much. Total aero drag is cd x frontal area, so that's where the vast majority of the aero load comes from, and while separating the airflow from the body is important as well, the body is designed to do that with the bed uncovered and the tailgate up.

And you're getting a hand calculated 30-32mpg? Congrats.

 

[Waldo Pepper]

I was wondering about that, because I just noticed the lip today and besides with 5'.5" bed the cover wouldn't make much difference anyway even without excellent aerodynamically designed truck.

My mpg is so damn accurate that it amazes me, thought my 05 MB E300 diesel was accurate always within 2-3 mpg above calculated, but my truck every time I've checked is within 1-2 mpg below actual and as a rule now I just go by gage only checking every other tank or two. The electronics in cars and trucks these days has improved so much in abilities and accuracy in the last 15 years that it's really staggering to me.

 

[kmcdonough]

I got mine with 3.92's with the idea that I may go to an oversized tire. The revs at 75mpg are still close to 2k. Maybe a 2wd tradesman could get away with 3.55's but the extra weight of these loaded 4x4 trucks do better with the lower gears...IMHO

In the end, I wound up buying an outdoorsman quad cab with the 3.92. It upped the towing and is pretty quick when accelerating. I live in a hilly area so I think it was the right decision and worth the sacrifice of a couple mpg.

 

[Waldo Pepper]

I didn't want that low of gearing because I figured I'd never tow more then couple thousand pounds or at most three thousand pounds. I probably hit 3,000lb mark moving using a friends trailer I borrowed. I still don't own a trailer and only reason I would need one is to take my small 1,900lb tractor to the shop for some reason, but I have a couple friends with duel wheel trailers with brakes I could borrow for that if needed or just pay the $75 pickup and delivery fee.

 

[Flash1034]

3.55 currently averaging 22.1 US MPG on fuelly but...winter fuel, winter conditions, idling and warm-ups, plus I drive normal speeds with traffic not trying to set a mileage record. I will expect 26 - 27 MPG highway again come summer. My towing with this truck will be either a 2000 pound enclosed motorcycle trailer or a 2000 pound boat so I wasn't concerned with finding a truck with 3.92. I am however looking at a 4" lift with only slightly bigger than stock tires so I hope I don't regret it after that. I doubt it.

If you're going to lift it and put bigger tires on it, I'd go with the 3.92 gears.

Flash

 

[Waldo Pepper]

I asked a guy the other day how his 04 Hemi that he bought about same time as I bought my truck was doing, his is equipped about same as mine except Engine, even 3:55 gears and it's same damn color. His best at 55mph is 18mpg and at 72mph says he averages 12mpg if not towing his bass boat or coming over my way in the hills to go to the Lake of the Ozarks then he says he's down to about 9mpg for the trip. What gets his goat is I can tow his rig w/o any real loss of mpg's or power. I let him borrow my truck for a couple days while his truck was in shop for repairs from accident. The insurance provided a car but he needed a truck to take his boat to L of O for weekend bass tournament, and came back loving the truck. I had told him before driving to never full throttle on takeoff if in a hurry, just at most about half throttle until RPM comes up past the 2200/2500 mark then hammer down. He said he is going to trade "up" to ECO Diesel for '16 model, only thing was what the DEF expense, but he said if a gallon only goes about 800 miles per gallon it still only adds about 1/2 cent a mile cost to the current local per mile cost of 10.4 cents per mile @ 25mpg average vs. 24.6 cents per mile @ 10mpg average. Local RUG is $2.59 and diesel is $2.54 a gallon.

Until he whipped the figures on me I had never actually figured the cost per mile to drive the truck other then guess it at about 40% less if cost of fuel wasn't more then 10% more the RUG. Wow was I off!!!

 

[Roc Doc]

In the end, I wound up buying an outdoorsman quad cab with the 3.92. It upped the towing and is pretty quick when accelerating. I live in a hilly area so I think it was the right decision and worth the sacrifice of a couple mpg.

I would seriously doubt that there is even a 2mpg difference all else equal. At 75mph I'm at 1916, whereas a 3.55 truck will be at 1735. That 10% reduction in revs pales in comparison to the aero load which of course would be the same for both trucks.

 

[phxpsd]

We have 3.92 and on a run from Phoenix to Colorado Springs we averaged 27mpg running an average of 77 mph.....empty

 

[JosephGage]

I decided to go with 3.92 because even though I got the Outdoorsman with crew cab and short bed I noticed that that gear ratio came std with the longer bed. I also noticed that in all my years driving diesels as long as they are within their ideal torque curve (virtually flat on these eco diesels from 1800-2600) mileage difference should be unnoticeable. Indeed I average 21-23 in normal driving and 28 on the highway, 18 towing 5000 lb TT.

 

[Waldo Pepper]

I drove a 3:92 enough to see there was a difference in MPG's and have a nephew with a 3:92 in his and his highway MPG averages about 2MPG less, but around town basically the same. He wanted a lower gear ratio because he tows an enclosed tool trailer 5 days a week, and has a bass boat and camper trailer. His truck has never given him a problem other then the factory tires wore out and replaced at less then 20,000 miles just like mine.