It gets into 8th gear. I just topped it off, took a highway run, calculated after topping off again, at same pump. Its exactly what the comp is showing. I also have 40pounds in the tires.Does it get into 8th gear? Are you watching the tach to see if it consistently shifts at the same RPM? Even if there is no code thrown you should have the dealer check for any that might be stored. There is certainly something wrong. As asked before, do you reset your MPG at each fill up? Are you calculating the mileage manually as well to verify if the car does it accurately?