A common mistake is to not reset the Fuel Economy Meter on the trip computer. The computer is calculating everything (Sitting at stop lights, flooring it to get on the highway and also cruising speeds). I have a 2500 High Roof V8 and get 15.5 - 16.5 on the highway. Also, actually do the math and divide the miles per tank into the gallons used (like we used to do in the old days before computers). It has to be better than 11.8 MPG's....