Given the increased amount of data traffic in in-vehicle networks with more systems now included in vehicles, enormous amounts of research have recently focused on providing reliable and inexpensive quality of service for in-vehicle communication systems. To support high-speed data processing while maintaining backward compatibility with controller area network (CAN) for in-vehicle communication systems, the turbo controller area network (TURBO CAN) was proposed, combining the standard baseband CAN signal with a passband modulated signal. In this paper, we demonstrate both an improvement in the performance and the validity of the backward compatibility of the TURBO CAN system. To be specific, we provide passband channel modeling of the CAN system based on the channel gain and the noise characteristics as measured in an actual vehicle environment. Using these measurements, we also provide proper criteria for determining the power constraint of the passband signal to achieve a highly compatible system by analyzing the channel capacity of TURBO CAN as well as the bit error rate of a standard CAN receiver.