Can someone educate me on which rim width is most commonly referred to? If someone says, "X brand of tire measures 44 mm wide on a 20 mm rim," do they usually mean the internal width inside the rim? Outside?

My vintage Araya RM-20 rims on my 1980s Bridgestone measures 20mm on the inside (from lip to lip). Am I right to infer that the inside width is the appropriate metric?

I've seen rim width expressed both ways, internal and external, and I've seen both measurements used often enough that I won't make any assumptions.

Internal is the relevant number for safety. Tire Sizing Systems

Tires see only the rim's internal width. Consumers tend to think about what they see, which is the rim's outer width.


There is a rough formula that relates increasing rim internal width and increasing tire width after mounting and inflation. About 3 to 1, so a 3mm wider rim (again internal) will result in about 1mm wider a tire.


But do understand claimed tire widths are not written in stone. The easiest example of that is the fact that tire "sizes" as marked on a sidewall are only labels, NOT actual measured sizes. Tire casings stretch with both pressure and age a small amount. Rims can and do also get pried wider with high pressures. Tires with significant tread can have the tread wrap around the casing far enough so that the tread is the widest point, so the same casing molded into different tread profiles can have different "running" widths.


Rim widths as found on rim labels are generally interior measurements. The "sizes" found in adds might be either internal or exterior and the addition of rim "height" can further confuse the consumer. Andy.

Further to this, and adding to the confusion, some tires will have marked with both a "common" size like 700C x 38 and an ERTRO size 622 x 40 or 622 x 37, for example, so it is uncertain which is the actual width.


Also, in a effort to market the lightest tire possible they will often make them undersized relative to their stated size. So a "25mm" tire might actually measure closer to 23mm.