I added a 1/2 bottle of Synpower oil treatment to my Dad's 98 4.3L Blazer and brought the cold tire pressure from 30 back to the speced 32 psi. He drove from CO Spgs to Denver and back and got over 30 mpg
Truck has 180,000 miles worth of pretty hard driving.
Sometimes I get concerned that something is broke on the truck BECAUSE it has always gotten unheard of gas mileage!