Kinematic models for skid-steer vehicles based on the locations of instantaneous centers of rotation (ICRs) of treads have been useful for motion control and dead reckoning. These models have typically assumed constant track ICRs because actual ICRs remain in bounded regions. However, this assumption neglects tread ICR variations during motion caused by dynamics. This paper analyzes the effect of inertial forces on track ICR positions. The study is based on dynamic simulations of a tracked mobile robot moving on hard horizontal terrain. Furthermore, a new kinematic model is defined in terms of three indices for sliding, eccentricity and steering efficiency. The proposed model allows to estimate actual track ICR positions as a function of inertia measurements and track speeds.