M u s t have knowledge.... (in a zombie like voice)
20/8x8.00 means a 20 inch tall tire, 8 inches wide by 8 inch diameter rim.
Rims are mearured in diameter and width, such as an 8x8 rim, 8x6 or even a 4x3. Many differing sizes.
It is possible to mount an 8 inch wide tire on a 6 inch wide rim, but it is not possible to mount a 6 inch diameter tire on an 8 inch diamter rim. You must match up the diameter numbers, height and width numbers can differ, depending on what you are trying to do.