Truckasaurus (2003 F-450 dually, 7.3 PSD, 4.88 rear) gets the following MPG:
12 MPG highway - not towing.
12 MPG city - not towing.
12 MPG highway - towing 15,000# fifth wheel.
12 MPG city - towing 15,000# fifth wheel.
If I use the "loud pedal" to go up from 60 to 70, I get 10 MPG. With a 4.88 rear end, the engine is spinning pretty fast at that speed, so I don't like to do it.