I have a 2006 Duramax (lly) with the Bully dog chip and i can get 22 out of the truck on the highway with the chip turned to the second highest level and unloaded.... 24 is a little bit of a stretch i am thinking but i have seen 25 from time to time if i am really light on the throttle and going mostly down hill but as soon as you get to a traffic light you are right down to 18-19. Idle time kills the millage.... Can watch it right on the economy/flow meter gauge.

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1987 300D 180k
1985 300SD 330k Helga
1992 300D Lola 269k- Sold
1986 325 coupe Bessie >300k Original engine(ODO Broke) sold
2006 2500HD Druamax Linda- Tows boat and looks pretty in garage thats about it