26" | 650A | 590mm
The only reliable way to match up a 26" x fraction bike tyre is by using the ETRTO number (European Tyre & Rim Technical Organisation).
The first number refers to the width, and is not critical.
The second number refers to the bead seat diameter, and this must be matched accurately. This number is usually on the side of the tyre and looks like, say 37-590.
French Size ERTRO Inch Sizing
650A 20-590 26 x 3/4