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Discussion Starter #1
I have a 2013 1500 CC with 5.7, 8 speed and 3.21 rear axle ratio. Towed my new boat last weekend on the interstate and got an average of 7.9 MPG. Reasonably flat interstate and running about 70- 75 MPH. The boat with fuel is about 6,500 lbs. and this is about the max GVWR for this truck with this gear ratio. I am considering replacing the rear end with a LSD and 3.92 gears which would raise the tow capacity to just over 10,000 lbs. I do expect to tow a camper in the future and that rig will definitely weigh over 6500 lbs, maybe up to 10,000 lbs. I like the truck and don't really want to trade it for a 2500 or diesel.
Question for those that tow with this set up and 3.92 gears; What fuel mileage do you average on the interstate? Is it worth the $900 to swap the rear end to a 3.92 for higher tow capacity and will the 3.92 gears help or hurt the fuel mileage?
 

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Hydrasport,
Have same config on a 2011 though 6 speed transmission, and pull a travel trailer maybe 6000 lbs. After logging over 12,000 miles towing this, I will get around the same mpg at 70-75. On a calm day, I see around 8-9 mpg at 65 mph, and 10 mpg at 60 mph. Depending on winds I adjust - a tail wind, I go faster, etc. I also tow up mountain passes, and have no issues, though would prefer the 8 speed tranny, higher gears. For example, can tow up the "Ike gauntlet" 7% grade at 50mph around 3.4k rpm.
 

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No, same as your 3.21. Meant same config as your vehicle except the transmission is a 6 spd.
 

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There's a lot of variables. I have a 16 5.7 8 sp. w/3.92's and get 12-13.5 towing my boat at 70+ mph on the level.
BUT stock, I got 9.5-11 towing the boat. Bit lighter boat but not much 20' Cobalt that sits up higher on the trailer.

3.92's will hurt mpg at 70+ unloaded up to 3-4 mpg.

Is your truck leveled or lifted at all? Highway or AT tires at all? Air dam still in place? Does the rear squat when towing to where the truck is level or nose up at all?

What gear were you running in while towing??

Your 7th gear = my 8th gear, your 6th gear = my 7th gear or very close. I generally am in 7th or 8th @ 70+ mph on I 95 on mostly level at close to sea level.
 

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Discussion Starter #6
What do have done to your truck to get better mileage than when it was stock?
I was stuck in 7th gear and only hit 8th on the downhill interstate overpasses.
I have air bags in the back that keeps the truck level when towing. Truck is level but about 1" higher than stock, front and rear. Factory air dam. I do have a "Superchips" tune and I did not change it from the "93" tune to the tow tune before pulling the boat last weekend. Truck has shorty headers that I installed to solve the broken manifold bolt issue. The cam and lifters were replaced last year after a cam/lifter failure and the MDS was tuned out. Those mods should not hurt the tow mileage but I will change the tune to the tow tune next week and see if that helps.
I want the limited slip rear end as much as a better gear, so that can all get taken care of if I swap the rear end.
 

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Level will kill your mpg.
I think the biggest thing was adding some rake and a partial lexan belly pan from the air dam back to the trans under the motor, other than that the only thing I did between trips was remove the high flow air filter and go back to a mopar stock filter.
But mileage jumped up on that trip and has stayed the same ever since.

Do you have a chrome bumper or painted? If chrome like mine (Outdoorsman, Big Horn, etc.), many if not all of the air dams will fold under in the center at 70 mph.
Next time on the interstate, look at a gen 4 Ram with a chrome bumper coming in the other lane - you will most likely see the air dam folded up in the center, which lets the air just ram into the engine crossmember.
My original Outdoorsman air dam is much shorter than most and the truck sat higher in the front than most - both allowed air to hit at high speed killing mpg. Stock I never got better than 17 mpg @ 70-74 mph empty.
I can now get close to or over 20 mpg but it is still very wind dependent.

MDS does help mpg even towing when going long distances, the mileage shoots up on every little downhill and it does add up over time.

I removed my skid plates prior and the lexan pan does double duty, I hit a semi truck tire recap carcass (at least 3/4 of the recap, it was huge) at 73 mph at night and the air dam and lexan pan took the hit. Air dam came out unscathed (it is reinforced behind it to keep it's shape and support it) and the lexan took the brunt of the hit. It kept the tire under the pan and let it pass under and spit it out of the rear of the truck. I did have to pull over and take the pan off since it knocked it loose and it was fluttering and hitting the pavement when at speed ( all of 5 minutes and 4 nuts) but it didn't shatter and I could see no damage to the truck or even any evidence that it hit anything else other than the dam and pan.
As hard as the hit was, I was expecting to see parts and or fluids coming out.
 

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Actually being in 7th and only 8th downhill isn't bad with 3.21's.
I am usually barely in 8th. The reason I say barely, is if there is any wind at all it will stay in 7th for long periods if I leave tow/haul on. It will go back to 8th if I shut tow/haul off, until an overpass at least.

I also have pretty much all season highway tires vs. any AT tire - General grabber HTS 60 tires in 275/60/20 size.
I pretty much only use the truck for highway travel.
 

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"I want the limited slip rear end as much as a better gear, so that can all get taken care of if I swap the rear end. "

2WD?

I don't think you would regret it but expect to take a hit on unloaded highway mpg.
 
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