We have two:
* 1997 Ford CF8000 ExpeditionVehicle with stock 505ci 250/800
* 1996 Dodge 2500 4x4 stock 359ci
Our ExpeditionVehicle is 14,000# across the scale.
It tugs a toy-hauler I built on a commercial chassis, about 14,000#.
We are in the slow-lane at 52mph.
We rarely get better than 14mpg.
The Dodge gets around 32mpg at 76-80mph across Idaho and Montana.
Plodding at 55mph, it drops to 20mpg.
Probably around 7,000#.