There are several ways to mark bicycle tire size, but the easiest way to find new tires that fit your bike is with the so-called ERTRO reading, which is based on millimeters. The first two numbers of the number tell the width of the tire and the next three numbers the inner diameter of the tire. For example, 47-622 means a 47mm wide normal 28″(622mm) tire and 47-559 a 47mm wide 26″(559mm) tire.
You have to be careful with markings based on inches, because tires and rims marked with the same inch size have different sizes. For example, the 26" rim used in the Helkama Oiva and Aino has a millimeter size of 584, and the 26" used in mountain bikes is 559. Also, a tire marked 28 x1 5/8 x1 1/2 (44-635) is intended for a rim of a different size than 28 x1 5/8 x1 1/2NL (40-622), despite the seemingly similar inch size marking, tires of these different sizes do not fit each other's rims.
You can usually install tires of different widths on the same rim, as long as the diameter remains the same. For example, instead of 40-622 tires, you can usually install 32-622 tires. When choosing tires of different sizes, however, the width of the wheel rims and forks and the space under the mudguards should be taken into account, as generally wider tires are also taller.
In Finland, the most common inch sizes and their equivalents in ERTRO readings:
- 12 1/2″ = 203
- 16″ = 305
- 20″ = 406 (children's bikes and BMX bikes)
- 22″ = 484 (old Jopots)
- 24″ = 507
- 26″ = 559 (mountain bikes and most modern youth bikes, etc.)
- 26″ = 584 (Helkama Aino, Oiva and Jääkäri. Also SA-INT wheels)
- 27.5″ = 584 (27.5″ mountain bikes, also known as 650B)
- 27″ = 630
- 28″ = 622
- 28″ = 635 (old, often Swedish standard wheels)
- 29″ = 622 (29″ mountain bikes)
INNER TIRES
Inner tires use the same marking methods as outer tires, but the sizes are "stretchy". For example, a 32/47-609/642 inner tire fits outer tires with a width of 32-47mm and a diameter of 609-642. That is, the inner ring mentioned above works for e.g. to the common 40-622 outer tire in hybrids.
There are three types of valves in inner tubes. The familiar car valve (needle/schrader valve), traditional quick valve (dunlop/woods) and presta valve. The presta valve was traditionally used on road bikes, but today it can be found on mountain and hybrid bikes as well.
The presta valve is thinner than car and quick-release valves, and its "soul" must be turned open by hand before pumping. In many cases, it is not possible to change inner tires with other types of valves to a bike with presta valves, because the hole in the rim is smaller.
TIRE PRESSURE
The recommended tire pressure for each outer tire is usually printed on the side of the tire, there are no pressure recommendations for inner tires. In general, wider tires have a lower pressure, and the higher you go to narrower tires, the higher the pressures.