[Steeveedee]

The truck in my avatar and signature gets 10 MPG in mixed driving. That shell sticks pretty far above the cab. With the old shell it was more like 13 MPG in mixed driving. Your rear gear is potentially the main culprit if it is a 4.11 or 4.56.

And the engine was rebuilt over 100k miles ago, so that isn't helping the mileage. Though it gets off the line nicely, it does consume oil, about 1 qt in 1000 miles.