Abstract An efficient method to determine optimal bushing stiffness for improving noise and vibration of passenger cars is developed. In general, a passenger vehicle includes various bushings to connect body and chassis systems. These bushings control forces transferred between the systems. Noise and vibration of a vehicle are mainly caused by the forces from powertrain (engine and transmission) and road excitation. If bushings transfer less force to the body, levels of noise and vibration will be decreased. In order to manage the forces, bushing stiffness plays an important role. Therefore, it is required to properly design bushing stiffness when developing passenger vehicles. In the development process of a vehicle, bushing stiffness is decided in the early stage (before the test of an actual vehicle) and it is not validated until the test is performed. If it turns out that vehicle performances are not satisfied in the test, another test with bushing changed needs to be conducted, which requires additional costs. Several tests are usually performed to identify bushings which achieve target performances. In addition, the decision of bushing stiffness is complicated since there is typically a conflict between requirements for bushing stiffness from various vehicle performances, such as ride, handling, noise, and vibration. Therefore, in the design stage, the validation of bushing stiffness is desirable to save costs of the vehicle development and ensure the performances of the vehicle. In this paper, a novel optimization methodology based on a numerical approximation model is presented. This method is used to determine optimal stiffness values of bushings in a vehicle for improving the vehicle noise. By using the method, it is found that bushing stif fness is well optimized while reducing the noise. Introduction An automobile has been mainly considered as a convenient transportation for human beings. As the number of automobiles increases, the amount of fuel consumption also grows. Hence, major interests of automobile technology have been to improve the fuel efficiency. Recently, however, demand for emotional experience and psychological comfort toward automobiles greatly increases. Namely , the perception of a vehicle changes from a transportation to a space which provides comfort and quietness. This trend will grow as autonomous vehicles appear to be popular. In order to satisfy the demand of customers, many studies for enhancing noise, vibration and harshness (NVH) performance are required. To develop the NVH performance, there are important steps in vehicle development to proceed, which are 1) design of suspension geometry, 2) evaluation of NVH performances along with refinements of body and chassis systems, and 3) bushing tuning. Among those steps, the bushing tuning is usually conducted at the final stage of the vehicle development. A bushing is typically composed of elastic rubber and metal pipes. Bushings in vehicle suspensions function to reduce vibrations in vehicles and finally decrease vehicle noises. Since bushing tuning does not require modifications of the body and chassis, it tends to be preferred. Several studies show that NVH performances were improved by bushing tuning, especially changing bushing stiffness [1]. Due to the effect of bushing for NVH performances, bushing has been consistently investigated in recent years and various types of bushing have been developed [ 2][3][4]. Although NVH performances can be enhanced by bushing tuning, bushing tuning requires trial and error even it is conducted by an expert. That is accounted for by the fact that requirements of bushing stiffness for NVH performances (such as road noise, idle booming, and lock up noise) can conflict. In addition, bushing stiffness has effects on other vehicle performances (e.g. ride and handling). Due to the complexity of bushing tuning, the cost of the vehicle development can increase. Therefore, it