Many factors governing wide variations in reported mpg (I suspect speedometer calibration is often the main one), but one not yet mentioned is extra drag from permanently locked hubs. The TJ and newer XJ front ends are always turning the carrier, ring gear, and front drive shaft. Vehicles with manually unlocked hubs or the old YJ/XJ vacuum disconnect passenger-side axle don't turn the carrier and drive shaft. In my own experience, this can account for 1-2 MPG(US) not to mention a much quieter ride.
Steve
ElAlumbrado wrote: