Found yesterday on the Canada Border Services Agency web site under the heading "IMPORTATION OF TIRES" a document
http://www.cbsa-asfc.gc.ca/E/pub/cm/...19-12-2-e.html that suggests that a zealous Customs inspector could make life difficult for someone trying to import tyres on their own, depending on the markings on the tyres. On the other hand, having their source document in hand could help if the circumstances are right.
Your interpretation may vary.......