I am trying to figure out every little way to save $$ on gas with my
truck.

2004 Chevy Colorado LS - regular cab - 2.8L i4 - 5-spd manual - 53K
miles

Stuff done to truck:
+1 size tires on the factory steel rims
Royal Purple Trans and Diff fluids
Mobil-1 5w30 extended life 15K mile oil in engine
K&N air filter

How I calculate MPG - fill tank, set trip meter to 0, drive till
empty, fill tank again, and divide mileage driven by gallons to fill
tank.

My current MPG numbers:
70mph Freeway, with A/C - 26.x
70mph Freeway, without A/C - 27.x
55mph Freeway, with A/C - 27.x
55mph Freeway, without A/C - 28.x
55mph Freeway, drafting behind a big-rig, with A/C - 31.x

Any ideas on how I can improve even more?