Need some advice. Recently bought an 02 Pete with a c15 6nz, 18 spd, 355 rears. Running 70 MPH I'm doing 1400 rpm. I was pulling a chip trailer getting 4.75 MPG. I'm now pulling my flatbed but the best I've seen has been 5.85 MPG. I've been driving 67 MPH at 1350. Does anyone have any suggestions to get my MPG up? I've only owned Detroit's this is my first go with a cat.