Originally Posted by
cjenrick
you need to realize that you want a number called 559, and not a number called 590.
one is for a 26" rim and the other i suppose is for a 650B rim.
Nominal 26" tires are a hot mess. Ignore everything except the ETRTO tire size designation, which will look something like this: "38-559" where the first number is the nominal width of the tire in millimeters and the second number the bead seat diameter in millimeters. Rims can accommodate a range of widths, so the first number only matters to determine if your frame has sufficient clearance for the tire. Anything narrower than the current tire will work; anything wider, you'd better check how much clearance you have. The second number is the important one, in that it must match your current tire, or the new tire won't even mount on your rim.
N.B. 559mm is the common bead seat diameter for "mountain bikes." 571mm is sometimes referred to as "650C" and may be found on road bikes for small riders. 584mm is "650B" or "27.5" and is becoming a popular choice for gravel bikes and people looking to run wider tires on road bikes. 590mm is "650A" or "26 x 1-3/8" and is commonly found on British and Japanese 3-speeds.