Expect no higher than 18.5-19 on the truck you described, and that is running 72-75 on the highway. You can make the numbers better if you drive 60-65 but even the fords and chevys will pass you at that speed.

 

04.5 600, 2wd, 3.73's, nv5600

12k on the clock, the computer tends to say 18. Improves on the freeway when I'm under 70 (2k rpm's, no faster, seems the best mpg), I've watched it hover 20-22 on I-5.

Tim

 

i hane a 2005 3500 gen 3 with a 6 speed i normally see 18.5 to 19.5 with no load and not over 70mph usually 65mph and speed above 70 makes a drastic difference

 

my 05 6speed dually 2wd gets 18.5 easily at 65-68 70+ drops about 1mpg only have 11000miles not quite broke in yet expect better

 

65 mph at 2000rpm is good for towing,should max mpg and no lugging other than major hills,I have the 5600 tranny with the 4.10 and turn about the same rpm,my problem is I would like to turn 2000 rpm at 70 mph to maximize the mpg,the 3.73 would do that,as you can see none of us are real happy,we are always looking for something better!I pull a 6 horse tr most of the time and am quiet happy with the Dodge other than a problem with the front tires which was really my screwup I have been trouble free.Knock on wood!!!

 

Don't have a 6sp, but getting around 20 according to the computer on the hwy @ 60-65 (5000 miles so not broke in yet)

 

SuperDuty,

To answer the question about the fan clutch... The fan is powered by a viscous coupling, which always provides some amount of torque to the fan. But instead of the usual bi-metallic clockspring providing increased torque transmission with high heat, the PCM controls the application of the clutch. The benefit is that the fan always spins, so when the clutch engages, there is less wear than if the fan was stationary. Also, the fan shroud is mounted to the engine rather than the back of the radiator. This allows tighter tolerances between the fan tips and the shroud, for better airflow.

I'm sure someone does (or could) make a controller for the fan. But honestly, I doubt you'd need it. If you're looking to increase engine braking by forcing the fan on, you'd get much better gains with an exhaust brake. Also, don't forget that the PCM will probably lock up the fan during the uphill climb anyways. It will most likely take the decent to bring coolant temps back down, and trigger the PCM to release the fan clutch.