He must have somebody pushing.
I've never seen a Chevy truck get 21... downhill with a wind behind it. I've managed 19.5 per tank if I take it easy on a long trip. Most of the time it's more like 17.
Nope...all me.
I should mention that I folded my mirrors in and I did run the vent with the moonroof cracked (airflow improves). I have nitrogen in my tires as well. It was probably around 65-70 degrees.
Here is the route:
I was going up north.......
FWIW, my grandma routinely gets about 17.X in the city in her 5.3 Rainier. About the same on the highway as me with 3.73 gears but less weight.