I average 12.7 towing a 16,000 pound high profile fiver running 60 - 63 on the interstate. I have 4.10. If I keep the speed down to 55, I get around 13.5, but find I need to drop out of 6th any lower than 55. Love my 4.10, except when running solo. Get about 23 at 50 - 55, 21 at 60, 20 at 65, 18 at 70, you don't want to know at 75. That is the penalty for the 4.10, but when towing heavy it is essential in my opinion.

Bob

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2003 3500 Laramie Doolie HO 6 Speed, 4.10 LS, Tow Pkg, Line-X, Air Safe 25K hitch, Jordan Brake Controller, RDS In-Bed Fuel tank/tool box combo, Jacobs Brake, Bremen boards, pyro, Fuel Pressure, RamAir III
Carriage Fifth Wheel, 36', Triple Slide, Sat, 6.5 Onan Marquis Gold genny, ProSine 2000w inverter, MORryde IS suspension, MORryde rubber kingpin

I think with 3.55 gears, 1700-1900 is going to get you 60-67 or so, I think. This a litttle bit fast for optimal mileage with a big aero drag load. This gearing seems fine for no trailer or a utility trailer.
AS you start getting over 55- 60 tops, your mpg falls off. With my 4.10 gears, I can do 52-58 at those rpms. I normally get 11-12 with my big 14k 5er, less with head winds. Pulling my utility trailer, it gets 15-16, loaded or not, at 65-70.
If you have the time and slow down, mpg will pick up. Maybe with 3.55, 60 is as low as you can go & still be in a decent torque range.
I am not disappointed in my 4.10's. For pulling a 5er, they are ideal, at least in Gen 2. I can get 20 mpg at 55-60, empty. A good day for a 7k truck, I think.
Airing up your tires on your trailer to max may help. I run 80/75 all the time on the truck. My fillings are in tact!
Sometimes trucks will get worse mpg from dirty sensors, or sometimes for factors no one can figure.
At the end of the day, if you get more than 10 with an aero load, life is good.
My 1984 P30 Motorhome,454, got 4-5 when I got it used. Best was 6-7 with headrs and other performance mods. 10 seems like heaven compared to that.

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2002 3500 4x2 QC SLT/Laramie HO 6 Speed/4.10 LS, camper/trailer grp, Silver;Bilsteins;K&N;Vulcan Kit, DiProcol FP, Boost, & Pyro,GoodYear Wrangler Silent Armor tires, 20k PullRite, 36' Alfa Ideal, 13k, 32' Mobil Scout, 18' 12k Uttility trailer

4.10 gears do NOT get you good mileage. This is why you can`t expect high MPG when towing. High number gears get the load moving easier, and keep the high end speed of the vehicle lower than Low number gears. More rpm at low speed= moving the load easier. Now, having said that, I have to ask why anyone would want 4.10 gears behind an engine that creates torque out the wazoo at LOW RPM. Unless one is to assume that you are going to be using the vehicle to pull outlandishly heavy loads (sled pulls). In which case don`t cry about the mileage pulling a camper. Just my 2cts.

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2003 Laramie 3500 Quad cab Long bead, CTD
NV 5600 Totally stock.

i tried that going under 65 towing my 06 does worse then when i run 70

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SIX IN A ROW BUILT TO TOW,CUZ WELL WE ALL KNOW , HAVING A V-8 AIN'T THAT GREAT Dodge trucks that havent made life expectancy
96 3500,-175k 98.5 3500 -164k ,98.5 3500-275k, 01 3500-140k, 03 3500 -155k, 07 3500-68k waiting for possible P-Pump conversion and now I'm killing the 06,3500-116k as of 10-01-09 and counting and yes i make my trucks earn a living buy towing professionally

I highly recommend a air dog fuel system!

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2004.5 2500 Diablo Predator, airraid cold air intake, 5 inch magnaflow ss exhaust w/ 7 inch tip, air bags, smoke cab lights, pioneer touch screen, 2 inch leveling kit, more to come

....... All you guys about 10 to 13 MPG pulling anything....

With the new fuel blends, EGR's, Crappy Aerodynamics, what did you expect? 4.10 Gears will help with the 1 to 60 MPH Towing. After that she goes down hill real fast. Just be happy that you can pull that 12k to 21k up that 6% Grade and live to tell your family your truck gets junky fuel mileage........ Oh, by the way------ Try 6K with a HEMI -------Once I get 3K behind me I am at 11 MPG HWY.........5K Boat with T-Top = 9......... 5k boat with T-top and E10 Gas = 8.......... I want my G56 , Quad Cab, 4X4, LB, Cummins BACK so I too can get terrible mileage pulling a 12k Fiver

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NOW: 2008 Dodge Ram HEMI...1500 BIG HORN White 4X4. Loaded! Proud Father of the ChunkyMunky!! Lexi-Lu

i used to run 454's i have no problem getting 7 to 10 towing what i tow. its when i'm empty that i care about mpg's

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SIX IN A ROW BUILT TO TOW,CUZ WELL WE ALL KNOW , HAVING A V-8 AIN'T THAT GREAT Dodge trucks that havent made life expectancy
96 3500,-175k 98.5 3500 -164k ,98.5 3500-275k, 01 3500-140k, 03 3500 -155k, 07 3500-68k waiting for possible P-Pump conversion and now I'm killing the 06,3500-116k as of 10-01-09 and counting and yes i make my trucks earn a living buy towing professionally

Boy I read these post about fuel mpg's of the newer Cummings and it makes me glad that I don't have a new truck. I put over 10K miles on my old 1999truck this summer pulling a Montana FW (11K) from Newport Beach Ca to Grayland Washington, to Idaho, to Oregon, back to Grayland then to Reno, Las Vegas, Las Cruces NM, Back to Newport Ca, then Nogals Arizona, crossed the border for our trip to Puerto Vallarta. My worst MPH was 12.6, my best 16.1 and I averaged 14.8 most of the time. My speeds 55 to 65 MPH with 70 MPH in Nevada. I love my truck when I hear the low MPG reported from the newer trucks.

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Damage Controlman Chief, USN Retired --"I live to Fish"
1999 Dually 3500 with 2955RL Montana Fifth wheeler; Superchip; Airraid Air intake filter; Banks High rise intake; Pyro/Turbo Boost/Tranny Temp Gauges; 4 inch exhaust; Suncoast Torque converter, Valve Body and Flex Plate; ATS Co-Pilot; BD Boost Fooler and Fuel Pressure Light; Airdog 100 Pump/Filter

i found that towing the same weight, 10k covered trailer, with the '03 2x and 4.10 dully quad cab and the '05 4x with 3.73 dully quad cab from NM to CA and back got virtually the same mileage at highway speeds.

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2008 RAM 5500 Cab/Chassis, 4x4, 6.7 CTD, 6-sp automatic tranny, 4.88 axles, quad cab, no bed, stainless steel fenders on rear duellies, 20,000lb hitch plate from Buyers Products with 20,000 lb. pintel hitch and a Dodge hitch receiver attached

2005 Chrysler 300C AWD - my wife lets me drive it once in awhile

2006 ASV-RCV multi-terrain track loader. (my SUV)

I am in the middle of a trip with my fiver and according to the overhead I am averaging 11.1 mpg. That is after 1200 miles and from almost sea level to around 7000 feet over some very steep grades.
For most of the trip I am towing at around 60-65 mph, but there have been times on nice straight roads that I have towed a lot faster.

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2003 Light Almond 2500 QC SLT, HO, six speed, 4x4, premium cloth, tow package, fold flat floor, 3.73 anti-spin, keyless entry, chrome Mopar nerf bars, rear slider, Rhino liner, immobalizer, Mopar mud flaps, BD short shift, ultrasonic backup system, Scotty 3 airfilter, White Knight backup lights flush mounted in rear bumper, Diprocal gauges, Jordan Brake, Icom 2/440m. 315 Yokahoma tires, Pacbrake, Carli suspension, on board air, Road Armor bumper, Dynatrac hubs. MM 12k hydraulic winch

Towing at 65 into a 30 mph headwind is aerodynamically no different than towing in calm conditions at 95 mph. The engine is spinning at a lower rpm (at 65mph), but the aerodynamics still suck. You burn more fuel overcoming the drag created by the airflow. The faster you go (or the bigger the headwind) the worse it gets. That's just physics...

I look for visual clues (trees, flags, waves on nearby water) to help judge windspeed and direction as I'm driving. If the gigantic Perkins flag is blowing right at my windshield, I expect worse mileage. I just hope that same wind will still be blowing in the same direction on the return trip!

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'05 3500 Dually 6sp w/Texson 800 slide-in camper...a significant towing upgrade from the old 'burban, and a helluva lot more comfortable to sleep in!

'81 CJ-8 Scrambler...caged, twin sticked, locked and loaded!

I am averaging 12-12.5 pulling 38K at 75mph. 4.10's are killing your 75mph milage.
I put 22.5 on my truck because of the low gearing of the G56 transmission. Instant improvment on milage.

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06 BLK quad cab DRW 3500
TST/Smarty
Stage 1 CP3
Southbend 3600 DD
22.5 wheels
Dynatrac free spin kit
ATS 3000/5000 twins
Firestone Airbags
Pacbrake
Transferflow fuel tank/tool box
1" lift in front from Top Gun Customs
Srt side pillar with boost gauge
Bush wacker fender flares
boxliner, and custom gooseneck hitch