Part-time systems are the most basic type. They are also still the most popular type for using off-road. The can be shifted (mechanically or electrically) from two-wheel drive to four-wheel drive. This gives them the versatility to operate both on and off-road. When the four-wheel drive system is engaged, the two axles turn at the same speed. There usually is no center differential. For that reason part-time four-wheel drive should not be used on dry pavement. If operated for any distance on dry pavement tire damage and drivetrain damage is very possible. Because it operates off-road, the vehicle typically has a higher Ride height. This is to go over obstacles on uneven ground without damage. Even when operated in two-wheel drive on-road, this system will not get the fuel economy of a comparable two-wheel drive vehicle. Locking hubs are available on some models. They help to get better fuel mileage by disengaging the front axle differential.