On my round trip to Moab, Utah from Payson, Arizona I got 23.4
. . . that's roughly 1000 miles. I was pretty amazed. I weigh 240, my wife is 120, and my dog weighs 80 lbs. Now add two camera bags stuffed and clothes, etc for 5 days. No head wind or tail wind.
Started at 5,000'. climbed to 8,000 feet, and down to 4,000' . . . and then reversed. Mostly flat highway. I traveled the speed limit plus 2-5 mph over except on I-40 where 75 mph was just fine (speed limit). City mileage varies as I live in a small town. Seems to be around 15 to 17.
I used cruise control (adaptive) whenever possible and in ECO mode the entire trip. A/C on most of the way. Not terribly hot. Passed folks when necessary. I am not really a slow driver - about average?
I had replaced the original stock Goodyear Fortera with Bridgestone Dueler H/L Alenza Plus with a V speed rating (higher) and more weight - 41 lbs for the Dueler and 34 lbs for the Fortera. (thicker side walls, stronger tire). 260 and A/A rating.
As the two tires are almost identical in revolutions per mile (less than 1%) there had to be some other factors at work. Used regular grade gasoline. The Feds say 20 mpg with mid grade gasoline. The Bridgestones are pricey and the Alenza Plus is supposed to provide better mileage.
Harder rubber?? Tire pressure was 36 lbs cold - as specified. Transmission is 8 speed with Fuel Saver Technology (extra).
Summit, 4x4, 5.7 liter 4x4