Yeah I just put ~15 in the case since I've never pre-filled the filter 100% on a change.

stock tire 235 85 16 width = 235 mm.
divide by 25.4 to convert to inches = 9.25 in. (width)
multiply by aspect ratio (.85) = 7.86 in. (height)
multiply by two = 15.73 in.
add rim size (16) = 31.73 in. (diameter)

285 75 16? diameter = 32.83 in.

you traveled 3.5% further than odometer said.

your new numbers: 15.30, 14.89, 16.92

If you kept it under 65 with not much hot rodding you should get 17 which is what I get and very typical. I would venture to say that only your speed and driving style are going to have a significant affect on your mileage.

Yeah under 65 would be nice but on the interstate people were already up my azz at 67-70. 99% of the time I drive like an old man since after 10yrs of high insurance the last 5 of super low insurance are nice and I don't want to screw it up. Heck I'm happy with the mileage, If I wanted 25+ I'd of bought a hybrid and my theory is you could take two "identical" trucks that came off the line one behind the other with the same driver and load and not get consistant mpgs between the two due to some little variance that gets them close but not dead on. Now I gotta find a new radiator.