This paper proposes a new car transportation system, iCART II (intelligent Cooperative Autonomous Robot Transporters - type II), based on "a-robot-for-a-wheel" concept. A prototype system, MRWheel (a Mobile Robot for a Wheel), is designed and downsized less than the half of the conventional robot used in iCART (intelligent Cooperative Autonomous Robot Transporters). In general, it is very difficult for mobile robots such as MRWheel to move to desired positions without motion errors caused by slipping, etc. Therefore we propose a decentralized control algorithm for car transportation in coordination by using a leader-follower type multiple robot system. The proposed algorithm enables the followers to estimate and reduce the motion errors and then enables the robots to transport a car to a desired position, even if the motion errors occur. In addition, we discuss how the external force applied to each robot during transporting a car, such as an inertial and friction force, is shared among the robots, and we propose a model-based external force estimation and compensation method. The proposed control algorithm is applied to the system, and the results of car transportation experiment confirm its validity.