Extra tires can be referred to as spares, backups, supplements, or reserves. These tires can come in handy in situations such as a flat tire, tire puncture, or even in the event of a blowout. The tires can also be referred to as auxiliary wheels or emergency tires because they provide an extra layer of protection and safety. Other synonyms for extra tires include contingency tires, surplus wheels, and additional rubber. In summary, extra or spare tires are essential accessories for any vehicle and can be referred to using various synonyms depending on the context or situation.