The most basic type of four-wheel drive system, part-time four-wheel drive operates in two-wheel drive mode (usually rear-wheel drive) all of the time on dry pavement. Four-wheel drive can be engaged only off-road or when the pavement is very slippery because this system forces the front and rear wheels to travel at the same speed. Using four-wheel drive on dry pavement can damage the drivetrain and cause extreme tire wear. Virtually all part-time four-wheel drive systems also employ a two-speed transfer gearbox.