I flew to Houston, Texas to buy my 2010 F250 CrewCab 4x4 with 3.55 gears, then drove it 70-75 mph back home, which was 600 miles. Bone stock I averaged 16.9 mpg (took time to fill the tank to the brim & hand calculated the mileage) and saw the "Cleaning Diesel Filter" display come on 3 times.
Immediately added the 250 Tune and deleted the DPF. Made the drive to Houston and back 2 months later, same speed, same route and basically the same weather pattern.
I got anywhere from 17.1 to 17.4.
Fast-forward another 10,000 miles with the 250 tune and my highway average at 70-75 mph is still around 17.0 to 17.5 mpg. I have seen one tank that did 18 +, but I had a pretty good tail wind.
How much horsepower would it take to get 19 to 20 mpg on my truck: 400, 500, 600 or higher?
I have 3.73 gears in my truck and I have gotten 17.5-18.0 on a long trip with the 210 tune. Back when I was leveled out and on 35" tires, I ran 70 mph from Knoxville, TN to Atlanta, GA one night and hand calculated over 19 mpg with the 275 tune.