Traditional two-wheel drive systems have a differential in the rear axle. This is used for turning the vehicle. If both drive wheels rotated at exactly the same rate, the vehicle would have difficulty going around a corner. In a turn, the outside wheel has to go faster to keep up with the inside wheel (which turns slower). A differential allows one wheel to turn faster under these conditions. This means only one of the two wheels actually has power at any given time. A differential also prevents excessive tire wear. 