What you are saying about the factory tires being taller than ours is right.
My point is that if smaller tires are causing bad milage, than mine should be about the same as Danoflapper's
By what 1010tires.com is telling me, I came up with the same tire diameters as you. The 215 tire is only .25% smaller than mine is, while the 235 is 3.98% taller than mine.
I ran the 235/85R16 which is what is recomended on the truck's door tag, which you said and is right, but I never noticed a huge difference in milage when I changed to the 254's (which are at about the same height as a 215).
The point I tried to make, amd I typed it fast and maybe I got it off wrong, was that the smaller tires shouldn't affect the milage that much. But you are right in that he SHOULD have 235's on there. I wasn't trying to make you look bad or wrong, and I am sorry if that's how it came out.
With the math I did, it only changed .5 MPG.
There is something wrong with that engine...or his right foot lol