Classic Mean-Shift based tracking algorithm uses fixed kernel-bandwidth, which limits the performance when the object scale exceeds the size of the tracking window. Based on the analysis of similarity of object kernel-histogram in different scales, i.e. the Bhattacharyya coefficient, a theorem is found and proved i.e. the changes of object scale and position within the kernel will not impact localization accuracy of Mean-Shift based tracking algorithm. Using this theorem an automatic bandwidth selection method is proposed based on backward tracking and object centroid registration. The proposed method is applied to track vehicle changing in size with encouraging results.