Best possible mileage?

i got an 80 model short step bed D150 stock /6 4 speed od truck, my daily driver, gets 18 mpg around town and bout the same at 65+ on hwy, but bout 22 mpg cruising the back roads at 45 to 55!! and that not even trying!! no idea how its timed, ant ever run over head, ant ever pulled the plugs, thing just keeps going, out run the energizer bunny!!