Maybe you need to replace your bicycle's tires because they are worn. Perhaps you want better handling and performance when riding. Whatever the reason, understanding the sizing system for your bike allows you to select a tire that fits your rim and performs as expected in the anticipated conditions. On the sidewall, there are two numbers with an "x" between them. The first number is the size, or diameter, of the tire. The second number represents the width.
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Road tires generally come in the size 700c, which roughly equals 700 millimeters. Some bikes, especially triathlon bikes, are available with wheel diameters of 650, or 650c. Some older road bikes still have the 27-inch size.
Because of the high pressure and desired low rolling resistance, road tires come in narrow widths. Common road bike widths, in millimeters, come in sizes ranging from 18 to 23, with a typical road bike tire at 700 x 23.
Touring and commuting tires are generally wider than racing tires. This allows for a more comfortable ride due to lower tire pressure. The extra width also provides more stability. These tires usually range from 25 to 28 millimeters.
Due to the terrain and lower tire pressure, mountain bike tires are wider than other tires. In addition, to allow for more toe clearance, the diameter of mountain wheels are smaller than other bike wheels, which are generally 26 inches. The width of mountain bike tires usually range from 1.8 inches to 2.4 inches, but can be as wide as 3 inches.
For smoother trails or hard-packed dirt, you may select a narrow tire such as 1.8. More narrow tires are also suited for muddy conditions to prevent bogging down in the mud.
Wider tires work well for rock or technical sections, so there is more tire to make contact with the trail. For very rough rock gardens or other obstacles, wider tires such as 2.5 inches to 3 inches are available.
29ers and Cyclocross Tires
There are mountain bikes with larger than normal wheels, called 29ers. Although larger, 29ers actually have a diameter like road bikes. -- The first number on these tires is 29. -- Widths of 29er tires are similar to mountain bikes and are also labeled in inches. A hybrid of road and mountain tires, cyclocross tires are labeled as 700c and have widths indicated in millimeters like road bikes, yet are knobbie like mountain bike tires.
Read the Size Carefully
Sizes written as decimals, such as 2.75, are different than sizes expressed as fractions, such as 2 3/4. These sizes are different and are not interchangeable.
Some tires may display the tire size in the International Organization for Standardization, or ISO, system. In this case, both the diameter and width are represented in millimeters. An example is a 700 x 20c tire would be expressed as 20-622.