Your bicycle tire is worn out and you want to replace it. You come in and tell us you need a 26″ tire. We are happy to help meet that request and need some more information. For bicycles sold in the USA and Canada tires are marked with two size designations: the North American name and the ISO tire marking. ISO tire size designations are an accepted international standard. Virtually all bicycle tires carry them in addition to the traditional North American name. The ISO tire markings consist of two numbers as follows: 50-559, with the 50 referring to the ISO tire section with in mm and the 559 referring to the bead diameter.
Below are some examples of the name and ISO tire markings.
You shared that yours is a mountain bike tire and it is off of an older bike. If your old tire had the above references: 26×1.95 and 50-559 then we have the above tire for you. But, maybe you have an old Schwinn bike and your tire has the following ISO tire marking and tire markings: 47-571 , 26×1-3/4 S-7.
The first tire example of 26×1-3/8, 35-590 is typically used on older hybrid bicycles. There is also a Schwinn equivalent of 32-597 which is a Schwinn S-5 to 37-597 which is a Schwinn S-6