What exactly causes 4wd to be less fuel efficient than 2wd?

[all_about_size]

I'm a little puzzled. It seems to me that there could more two main factors:

1. The 4wd uses a bit of torque at rear during acceleration (just read about that). That helps to explain the 1 gallon less mpg for non-highway driving. But why it is also 1 Mpg less for the highway when the 4wd is not engaged (since pilot cruises at high speed)?

2. That leads me to think that the proportional 1g less mpg is due the weight of the differential gear, etc.? The distribution of torque to the rear does not consume more energy?

Any guesses or ideas?

Best,
SY

[rocky]

Vehicle weight increases plus additional friction losses of the extra driveline components.

[mudbog]

Even when 4WD is not engaged, the gears in the transfer case, propeller driveshaft to the rear, and part of the gears in the rear diff. still spin (when 4WD is needed, there are 2 clutches inside the rear diff. that are engaged). All those spinning parts need a little extra torque to get them up to speed and there is friction from the bearings and fluid in the transfer case and rear diff that needs to be overcome even when maintaining a steady speed.