[shovel]

Extra weight is extra weight, and extra spinning weight is extra spinning weight.


I'd be surprised if there was any consistent difference in mpg whatsoever between an AWD car in AWD mode and an AWD car with all of its drivetrain bits spinning but only 2 wheels actually receiving torque from the engine. The wheels don't care if they are connected to the engine by a shaft or by a planet whose surface links the front wheels to the rear, the net result is the same parts spinning and causing drag.

As a side note, early (at least through 2001) 5-speed Foresters (and probably other soobs) directly transmit engine torque to the front wheels and only utilize their viscous coupling to link the front wheels to the rear. What this means is on those vehicles you can simply unbolt the rear diff & drive shafts and remove them entirely from the vehicle with no consequence other than loss of AWD. The output yoke on the transmission will just spin and the front wheels will continue to receive 100% engine torque.