system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do trucks/vehicles that have optional 4wd get worse gas mileage, even if the 4wd is disengaged?
gpt
Added weight and more power loss in the drivetrain.

Instead of having a driveshaft going from the back of the transmission to the rear wheels, you've got the transfer case in between. You lose power due to friction in the transfer case, since you've got more moving parts involved. 

The front wheels are also going to lose power to the front drivetrain components, since you've added more spinning parts there (even when in 2WD mode).

All of these components weigh more than zero, which takes more energy to move around.