THE PR card application guide says that you need to get PR card to reenter Canada via commercial transporter like plane, train, bus or Ship.
See http://www.cic.gc.ca/english/pdf/kits/guides/5445E.PDF
It does not mention about personal Vechile like Car
Alos Fee Faq http://www.cic.gc.ca/english/information/faq/pr-card/pr-card-faq12.asp
Which says if you you need to satify Border officer of PR status by what ever means u enter canada including car and PR card is best way to do that. In case u do not have PR card COPR may work but officer may ask you questions to ascertain your PR status
So I think thats way we can say to have PR card is best bet, but by own car when can re-enter using COPR provided border officer is satisfied with PR status