The basic tire size is ETRTO
Whether road bike, gravel road, or MTB, tires for today’s sport bicycles are basically manufactured according to the European tire standard called ETRTO. The same applies to the rims of the wheels on which the tires are mounted.
Therefore, if you know the ETRTO size of the wheel rim, you can generally determine what size tire can be mounted.
For road bike tires, French standards such as 700C are more common than ETRTO, but 700C is equivalent to “XX-622” in ETRTO, while 650B, a popular gravel road tire, is equivalent to “XX-584”.
If you look at the bead portion of the actual 700C tire, it should say “700 x 30C [30-622]”.
How to find out the compatible tire size of a wheel
Check the manufacturer’s recommended tire size for the basics.
The latest wheels have detailed specifications on the manufacturer’s official web page. By checking there, you can find out the tire size recommended by the manufacturer for use.
As an example, let’s look at the Shimano WH-RS710 wheel.
SHIMANO C32 Tubeless Disc Brake Wheel Rear | SHIMANO
Shimano’s official page for the WH-RS710 states that the recommended tire size is “25-622 to 32-622”. As explained earlier, “XX-622” means “700 x 00C,” so we can see that tires in the 25-32C thickness are Shimano’s recommended tire sizes.
It also says that the width inside the rim is 21 mm, so we also know that the WH-RS710’s rim size is “21-622”.
Recommended tire size varies from manufacturer to manufacturer, but for road wheels, most manufacturers recommend a tire size that is approximately 4-12mm wider than the inside width of the rim.
However, in some cases, gravel wheels allow tires that are 40 mm larger than the rim size.
When the recommended tire size is not listed on the manufacturer’s official page
On the other hand, there are cases where the recommended tire size is not listed on the manufacturer’s official website.
For example, the specifications of Bontrager’s Paradigm TLR Disc, which is installed on TREK’s complete bikes, do not list recommended tire sizes.
Bontrager Paradigm TLR Disc Road Wheel | TREK
In this case, the compatible size is assumed from the rim standard.
According to the official website, “the inner width of the rim is 19.5mm” and “the diameter is French standard 700C,” so we can see that it would be “19-622” or “20-622” for ETRTO.
If we take into account the recommended tire size range for road bikes (+4 to +12 mm) as explained earlier, we can assume that the recommended tire size for this wheel is “23-622 to 32-622”, or 700 x 23 to 32C.
Another method is to use a formula to calculate the appropriate tire size based on the inside width of the rim. The formula is “Tire Width (C) ÷ Rim Inner Width (mm) = 1.4 - 2.4.” The closer to 1.4, the lighter the tire rolls, and the closer to 2.4, the better the cushioning.
Using the above formula to calculate the tire size for the wheel just described, the minimum is “19.5 x 1.4 = 27.3” and the maximum is “129.5 x 2.4 = 46.8”.
However, the shape of the rim is also important in order to fit larger tires. Especially in the case of tubeless tires, the optimum size of the inside width of the rim and the tire width is determined based on the compatibility with the bead, etc. Basically, it is safe to assume the recommended range for road tires if they are road tires.