The fan kicks in when the ambient temp is higher or under a load that creates more heat. The fan will take some power away but its temporary.

My 06 is getting 17-19 ave, on the highway I can get up to 22mpg (65-70 mph). Towing getting 12-14, tow almost 10K 5er. (68 mph)

Key is to keep it under 2K rpm's. Not sure where the sweet spot is on the newer 6.4's but you'll find it and I also think your normal driving ave will get better.

Oh yes, my 06 is programed using SCT XCal II with custom tunes.

 

Yes, that's definitely the fan.

The 08 trucks get worse mileage due to EPA concerns. The truck runs less timing than the 6.0L trucks and it has a Diesel Particulate Filter which requires regen about every 200 miles. During regen fuel is delivered throught he exhaust ports to super heat the DPF and clean it out. The changes in fueling and the regen process knock off the 2-4mpg extra you would see.

Some programming will add a little timing and pick up the mileage.

 

Thanks for all the input guys! If it is the fan, are you telling me it is not engaging when I'm driving around town? We have had a ton of 95 degree days in Denver and I have never heard the noise in town?!? It's hard to imagine the fan has only engaged when I have been towing to the mountains. Do diesels run that cool and don't need the fan??

 

Mine comes on at the strangest times....hot in Texas? Never

JP

 

Got a 08 with the V-10, I get the same occasional fan noise as you do.