Why Is Fuel Economy Lower With AWD/4WD?

My Answer: Two reasons: the extra weight (mass) added with the components and power loss due to the added mechanics.

A two-wheel drive vehicle normally churns power from the engine to the transmission, which then sends power to the drive wheels. The mechanical losses are minimized by keeping the transitions of power from the engine as simple as possible. This is one reason that front-wheel drive is more efficient than rear-wheel drive, as the tiny percentage of power lost to the drive shaft and differential to run to the rear wheels is not present on the FWD car.

Read about this at CarNewsCafe.com.