hi, someone gave me this wheel and i really want to use it on a project bike i have, but I cannot find a tire that will fit it. i have tried both an 18 inch tire and a 20 inch tire. the 18 is definately too small and the 20 is definately too big. i searched for a 19 inch tire but they dont seem to exist. no writing on the rim and the person who gave it to me had the same problem. any help please!!!