I don't know how accurate those digital overhead calc. are. But I took a trip with Ironman in his old '04.5 Cummin dually w/4.10 gears. I looked at the over head a couple times and it said 22-23mpg. His truck had a some kinda chip, airfiler and exhaust if I remember correctly.
I'm sure that over stated what he was getting...but it must of been high teens at least.