Having the fuse in or out, won't affect the fuel consumption. When the fuse is in, the transfer clutch is disconnected, but the rear wheels are still driven by the front wheels, through the road.
As long as the front wheels are not spinning, both sides of the clutch still rotate at the same speed. It won't make any difference if the clutch is engaged or not.
Driving with the fuse in FWD, means that the TCU can't engage the rear wheels, to prevent front wheel from spining. If the front wheels spin, the open front diff will be subjected to jolting as the wheels grip or spin. Many front diffs have been destroyed because of this.
Harvey.
