Many types of variable speed accessory drive systems have been researched and developed, but none have been adopted in a production car1,2. There has been an increasing demand on the accessory drive system due to the adoption of new accessories in passenger cars. Along with the increasing demand on the system, nothing can be done which would decrease fuel economy. So, the efficiency of the system must be improved. This has caused renewed interest in variable speed accessory drives.A compact V-ribbed (serpentine) belt CVT system for accessory drive (called CVAD: Continuously Variable Accessory Drive System) has been developed3. By applying the CVAD for the accessory belt drive system, fuel savings, increased vehicle performance in acceleration, noise reduction, increased life of accessories and V-ribbed belts, and a reduction in the size of accessories are possible. The performance of a prototype variator, the principal component of the CVAD, has been evaluated. A high power transmission efficiency of over 87% was obtained with a speed ratio of 0.6, a torque capacity of 40 Nm, and a slack side belt tension less than 400N. Good controllability of the belt tension was also achieved.