This is an original 1961-66 Ford F250 Hubcap, that snaps on the little nubs on the side of dome on the hubcap on the lug nut side.  There are other hubcaps in later years that snap on the outside.  You can see both measurements on the hubcap, Outside to outside is about 10 1/4, and inside lip that snaps on the nubs is 8 3/4". It is a used cap, but in really nice shape for its age. 

This is NOT for a F100.