Bicycle wheel & tire sizes are based off of a sizing system called ETRTO which stands for European Tire and Rim Technical Organization.
The first number indicates the width of the tire or rim while the second number indicates the inside diameter of the tire or bead seat diameter (BSD) of the rim. ETRTO measurements are done in millimeters so the term "54-559" translates to a tire width of 54mm and diameter of 559mm. 54mm is equivalent to 2.1" while 559mm is the same as 26".
A road tire ETRTO of "25-622" translates to a 25x700C tire with a width of 25mm and diameter of 622mm. The "C" in 700C refers to the height of the tire while the "700" refers to the diameter of the wheel using an old naming convention that no longer exists. 700 used to refer to the outside to outside diameter of the tire in millimeters. Since 700C tires have an inside ETRTO diameter of 622mm there is a difference of 78mm which means the tire should be approx 39mm tall on either side.
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