First let me say this ours has the 4/10 gearing. We had a Dodge 2500 HD, short bed that was not a 4/10. The biggest difference is with the 2500, 70 mph equaled 1500 rpm, while with our 3500 70 equals 2000 rpm.
So far I have only towed to South Dakota and back, Wisconsin and back, all around northern Michigan, couple times into Indiana and Illinois.
I get my best mileage traveling at right around 63 mph. My average is 9.3 when I keep it around 63. Drops to mid 8s if I do high 60s to low 70s. Worst I have had was mid 7's semi flat roads with a strong headwind.
I am always checking my average mpg......I never use the cruise control. I take advantage of downhill grades to gather speed and will shed some mph to keep my average up going up hill. I try to never be in a hurry and try to hold 63 unless I can get away with a little faster and not drop below my average.
Just an added note...….Gas Buddy has saved us a ton of money on Diesel. When I hit half a tank I check the price difference. Amazes me many times the difference is around 25 to 35 cents a gallon, high to low. I have seen it as high as 50 cents a gallon. I did travel 8 miles off the highway once to save 35 cents a gallon....I was empty