This has been my question, I tried to see if I could see or tell any difference whatsoever and notta, many in the parts industry seem to think it was a part# change only to represent different applications.Talking to my parts guy today he says 652 supersedes 651. If you try and order 651 it sends him straight to 652. Could be a different rubber compound or cords. Who knows what they did. Must be better, I guess...