Motors used in electric vehicles have a very unique and harsh running condition and environment-High efficiency is especially demanded in these two conditions; the acceleration needed upon start, and upon riding the expressway, which is about lOOkm/h to 120km/h in Japan. In order to increase the efficiency in these two conditions, shape and volume of the motor should be well thought out because of the limited space. The conditions are different with every vehicle but there is a need to find the best ratio of the diameter and stack length. In-wheel motors have the best transmission efficiency while having many other advantages because every wheel is independent. However, in-wheel motors are very much restricted in terms of shape and volume. Interior permanent magnet (TPM) outer rotor motors with different diameter and shaft length and numbers of slots and poles were simulated to sec the characteristics torque and efficiency of the motor. By optimizing the size and shape and also the electromagnet structure of an outer rotor motor, the driving range for electric vehicles increases. By increasing the diameter, maximum torque increased at a parabolic curve while by increasing the stack length the maximum torque linearly increased. By this, the maximum torque of the motor is approximately decided by the volume, diameter squared multiplied by the stack length. However, while a thin motor has a short torque constant area of the NT curve while, a thick motor has a long torque constant area. This results in a change in efficiency at different driving pattern.