Bicycle tires come in a bewildering variety of sizes. To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. Even worse, different-sized tires that were not interchangeable with one another were often marked with the same numbers!
Traditional Sizing Systems
The traditional sizing systems are based on a measurement of the outside diameter of a tire. This would usually be measured in inches (26", 27", etc.) or millimeters (650, 700, etc.).
Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. Here's how it works: Let's start with the 26 x 2.125 size that became popular on heavyweight "balloon tire" bikes in the late '30's and still remains common on "beach cruiser" bikes. This size tire is very close to 26 inches in actual diameter. Some riders, however were dissatisfied with these tires, and wanted something a bit lighter and faster. The industry responded by making "middleweight" tires, marked 26 x 1.75 to fit the same rims. Although they are still called "26 inch", these tires are actually 25 5/8", not 26". This same rim size was adopted by the early pioneers of west-coast "klunkers", and became the standard for mountain bikes. Due to the appetite of the market, you can get tires as narrow as 25 mm to fit these rims, so you wind up with a "26 inch" tire that is more like 24 7/8" in actual diameter!
A second number or letter code would indicate the width of the tire. (26 x 1.75, 27 x 1 1/4...650B, 700C...)
Does Point Seven Five Equal Three Quarters?
Note that the inch-based designations sometimes express the width in a decimal (26 x 1.75) and sometimes as a common fraction (26 x 1 3/4). This is the most common cause of mismatches. Although these size designations are mathematically equal, they refer to different size tires, which are NOT interchangeable. It is dangerous to generalize when talking about tire sizing, but I would confidently state the following:
Brown's Law Of Tire Sizing:
If two tires are marked with sizes that are mathematically equal,
but one is expressed as a decimal and the other as a fraction,
these two tires will not be interchangeable.
The ISO/E.T.R.T.O. System
The modern ISO/E.T.R.T.O. system indicated the size of the rim, not the outer diameter of the tire. This makes the ISO/E.T.R.T.O. system the most reliable guide to which tire will fit which rim.
The key ISO/E.T.R.T.O. dimension is a three digit number known as the "Bead Seat Diameter. "26 inch" tires will have ISO/E.T.R.T.O Bead Seat Diameters of 559 mm, 571 mm, 584 mm, 590 mm or 597 mm.
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