Updated
To be confirmed by PowerTred
In a marking such as 540/65R30, 540 is the nominal width in millimetres, 65 is the aspect ratio and 30 is the rim diameter in inches. R indicates radial construction.
Other marking formats exist. Copy the complete size and check the machine specification, rim, load and speed requirements with a tyre specialist.
Use Find a tyre in the header to search the exact size. A near match is not an equivalent, and a size match alone does not establish suitability.