The computers in the newer trucks lie about the mileage you get compared to doing it longhand, that is a duramax, powerstroke and cummins people I work with use. Usually the mileage is less when computed long hand. I have a 97 dodge 12 valve and running under 70 with the cruise on I get right at 20. Pulling a 29' camper down the highway at 75 I get 12. The only thing the computer generated numbers are good for is bragging to your buddies about how great of mileage you get.
