so about a month ago i found a dana 70hd that had 4.10's and LSD in it and swapped it with the 3.55 dana70hd open diff i had
for the most part everything was a direct swap the only difference was the speedo but i think it was because of the different year
the truck is a 98 2500 with the v10 and around town im getting around 12mpg with the new 4.10's and 245/75/16 tires
i installed a new trans temp gauge and took the truck on a test run with the new trailer i picked up loaded down with my crawler

left the truck in O.D. on the highway and the thing pulled great , no erratic shifting and didnt really downshift at all the whole time i was watching the gauge and temp never got above 182*
so im waiting to see how i do for mileage while towing , with the 3.55's it was 6.5mpg

 

That's good to hear. I'm getting my 4.88's installed tomorrow. How much does the trailer and crawler weigh?

 

truck is 5200lbs with a full tank of gas and the trailer is around 2000lbs maybe a little more
so somewhere between 7200-7400lbs total

 

your 2500 is 5200lbs?....hmmmm

when i went to the scrap yard last, when i was unloaded, and leaving...the scale said my truck was about 5350 lbs....and i have the 1500.....


sry just find this odd....

 

by "truck" i think he meant crawler

 

correct the crawler is 5200lbs , the dodge is 6000lbs

figured it was a given that i was referring to the crawler since someone asked me how much it and the trailer weighs

 

That's a big crawler lol. Those 4.10's are probaly like night and day.

 

I just did a test for towing in OD when I moved from Arkansas to Indiana 2 weeks ago:

Gas: 87 octane
Truck: 2001 5.9L extended cab with 4.10 gears (stock from off road package) a camper top. Mods in signature (Stock airbox, took off the mopar round)

TRUCK UNLOADED WEIGHT: 5600 lbs
TRUCK LOADED WEIGHT: 8900 lbs. Towing a motorcycle and trailer, probably about 1700 in the bed)

Towing and hauling 3,300 lbs of stuff traveling 65 MPH
In OD: 10.6 MPG for 15 miles (relatively flat)
Out of OD: 12.9 MPG for 15 miles (relatively flat)
In OD until I hit a hill, then took it off OD to stay at 65 MPH for 15 miles : 11.1 MPG average, instant MPG readout

I noticed that if you are in OD, you hit a hill, your speed goes WAY down and you need to press the pedal to the floor and you do not even accelerate until the tranny kicks down.

MORE DATA:
Higher AFR (I have a gauge) when you are chugging along in OD trying to keep a constant 65 MPH
Lower AFR when you are out of OD and much easier to stay at 65 MPH.

FINDINGS:
Mileage has more to do about the mixture than the RPMs.

 

kinda hard to compare 2 different engines and come up with the same conclusion
when i was towing in O.D. at 65mph (pretty much flat) the v10 had no issues keeping speed most of the time it didnt even feel like there was anything back there
rpms were at 2k , same place they are without towing but put it in O.D. im at 3500k
at the same speed on the same road
i cant see how that would increase gas mileage as you posted

 

Mine's not that way at all -- perhaps because I've got the Superchips towing tune always installed and/or the tweaks that go into the Mega Viper. My truck always disengages the overdrive unit right about the time I'd do it manually due to the seat of the pants feel, and I never lose more than 2MPH before it disengages. Usually there's no loss of speed at all, just the MAP sensor saying "Hey, we're working hard here".

The only time overdrive saves you fuel is when the engine is just loafing along. This is true of every overdrive in every vehicle that has one.

I don't mean to come across as a dick here: The better take-away from your little experiment would be: When there's a significant load in or behind the truck you should try to cruise at 2200-2400RPM where the engine more efficiently makes power. The A/F ratios you observed were just side effects -- you could just as easily have monitored a vacuum gauge or MAP sensor output, as all three of these things are indicators of engine load vs. RPM.