In this paper, the basic tractive characteristics and the optimization of the performance of four-wheel-drive off-road vehicles are analyzed from the point of view of tractive efficiency.It is found that the distribution of thrust and slip between the front and rear wheels has considerable effect on the efficiency of operation. Differences in theoretical speed and the type of coupling between the front and rear axles generally affect the thrust and slip distribution.It is shown that the optimum thrust distribution is that which can make the slips of the front and rear wheels equal. Only under such circumstances will the tractive efficiency of a four-wheel-drive vehicle be a maximum. This requires that the thrust distribution should vary with the weight distribution of the vehicle as well as terrain and other conditions. It is pointed out that the inefficient distribution of thrust and slip also causes excessive tire wear.The analysis indicates that the development of a new type of interaxle coupling which can provide the optimum distribution of thrust and slip between driven axles is of practical importance.