The only time I kept track of fuel economy, my 2002 GTS returned 21.6mpg on an interstate trip from Illinois to Washington. I wasn't a hypermiler then, just driving normally. The T-56 had a very tall 6th gear, keeping engine RPM at ~1900 at 80mph; slowing down a little would probably reap a couple mpg at least, with lower aerodynamic drag , lower RPM, and increased throttle opening. In the truck, with a slightly larger engine (8.3L vs. 8.0) and the same transmission (taller final drive and taller wheels/tires) but horrible aerodynamic efficiency, the difference between 65mph and 80mph was as much as 4mpg.
Somewhere in Wyoming:
