What's the gearing like on those trucks? I'm curious how the mileage would be if they were turning really slow, like 1500 RPM at cruise with a mild enough cam to keep the cylinders filling well at that speed? A 2000 Durango 5.9 I worked on cruised at barely 2000 RPM on the freeway at 75 MPH with that fat overdrive in the 46RE trans and still had plenty of torque for getting up hills and such.