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Every tire has its size moulded into the sidewall. In a size like 225/65R17, that last number — the 17 — is the diameter of the wheel in inches. That is also your hubcap size, because a cover has to match the wheel exactly.
This trips people up constantly. A 16-inch cover will not stretch onto a 17-inch wheel, and a 17 will not stay on a 16. It is the single most common reason someone buys the wrong hubcap online and has to send it back.
If the sidewall is too worn to read, check the sticker inside the driver's door jamb — the factory tire size is printed there too.