I don't know why some Tundra owners only get 12-13mpg. The difference may be multiple things that add up in the end.
Here's my truck:
2000 Tundra, ext cab with 2wd (less weight & drive train loss?)
I drive mostly highway
Lightweight, hard tonneau cover, flush-mount (1mpg by some reports)
87 octane with fuel injector cleaner every oil change
5W-30 oil
no nerf bars (less drag?)
August 2002, Poplar Science did a small, unoffical study to improve gas mileage on a pickup truck based on recommendations from a GM aerodynamics dude. Some of the suggestions were impractical, so you're mileage may vary on some of these mods.
1. Taped over parts of the grill, fog lights, ect. (was a diesel engine)
2. removed nerf bars
3. added hard tonneau
4. filled in space between frame rails and body on the underside
5. They set cruise at 55mpg (the faster you try to push the brick, the more mojo it takes.)
Like I said, some were impractical......
The results were the Silverado Ls 2500HD with a 6.6 turbo-diesel went from 17 mpg to 26 mpg...........interesting.
FYI, I've only had one K&N filter and that was on my T-bird. It dropped my mileage from 25 to 23 for over two years. Pulled the K&N out and mileage went back up. Go figure........it did seem to improve high rpm flow (above 4,000rpm). Granted, mine was only a study of one.
Good luck, and keep us informed if you try that frame-rail thing
Everett