Southpaw and Lars pretty much covered it. I know Fishchris' truck, because he did a TON of postings in the "Appearance Products" forum in f150Online when he was getting his truck.
The boat he pulls is very lightweight, something less than 1000# with the trailer. His truck is an SCab, not an SCrew, so it's somewhat lighter than ours. It's 2WD, not 4WD like ours. It has 17" wheels. I don't know what his gearing is, but I suspect it's lower than 3.73.
So, everything I know about his truck tells me he SHOULD be getting better gas mileage than we do.
I averaged about 15.5 mpg in just under 1400 miles on our last trip at 70-75 mph (hand calculated at each fillup). I get around 12.5-12.7 when we pull our 3600# trailer. My tires are 275/65-18 Goodyear Wrangler Silent Armors and they have a fairly conservative tread - I'm sure mine have less rolling resistance than yours.
Both you and I have more bells and whistles in our trucks than Fish has. For instance, we have climate control, and it's ALWAYS on in my truck. That consumes energy.
I don't trust either the Gryphon's or the truck's MPG displays. The truck's is always lower than the Gryphon's average numbers and my "hand calculated" results fall between the two.
One point - unless your loaded tire circumference is correct, your speed and odometer values will both be wrong. For my tires, I've subtracted 3% from the unloaded tire circumference. Using Bill's calculator, that corresponds to a 7%
rubber squish (since the wheel doesn't squish). I don't know what your unloaded circumference is, but you might try entering a value that is 3% lower for the tire size. (A larger number shows up as higher speed and more distance driven per tire revolution - and, when the Gryphon's speed readout is correct, the dashboard speed will be 2 mph faster, but the odometer will be accurate).
Finally, each truck is an "individual". Some will simply perform better than others and there's not a whole lot we can do about it.
Two more posts appeared as I was writing this "book".
The "fuelly", site was quite interesting, Southpaw. Looks like the trucks getting about 15 mpg are in the majority. And JWBFX4's remark on the tires is dead on!
- Jack