Can you guys all provide more vehicle info?
What gear ratios, transmissions, speeds the fuel mileage is veing judged, and what octane fuel and alcohol content of fuel in your area for these mileages you are getting?
My first 1999 Ram 2500 with 5.9 Magnum has 4:10 gears, four wheel drive, overdrive auto trans, and larger than stock tires. This truck always about 11 mpg at 75-80 mph speeds, and would barely reach 13 mpg at 65-70 mph.
My current 1999 Ram 1500 with 5.9 Magnum also has four wheel drive, overdrive auto trans, larger than stock tires, and I think it has 3:54 gearing. This one gets 13 mpg at 75-80 mph speeds.
My old 1993 D250 with 5.9 Magnum is two wheel drive, overdrive auto trans, 4:10 gears, stock tire size. That one never did better than about 14 mpg at 75-80 mph.
All my trucks have run regular unleaded when not towing, which is usually 87 octane with 10% alcohol here in Michigan.