4WD is not on all the time, under normal conditions, only the rear wheels drive the vehicle forward, while the front wheels spin freely. To activate a modern four-wheel drive system, the driver has to push a button or pull a lever to engage it.
source/image: Wonder World
But four-wheel drive isn’t meant to be on all the time so you have to know when to turn it on and when to turn it off.AWD is on all the time, and mostly used in cars, and the computer manages the system.
Just going down the highway, the system will send most of the power to the rear wheels, for maximum fuel efficiency, if the road condition changes, such as rain or snow, the system adapts and evenly distributes the power evenly to all 4 wheels, so you have maximum drive grip, with little to no chance of wheel spin.
2WD Cars are less complex than those with AWD or 4WD, and their simpler drivetrains mean improved fuel economy in the long run.In general, cars equipped with 2-wheel drive get better gas mileage than models that use all-wheel drive or 4-wheel drive.