The short and only honest answer is that it depends.

Each specific tire/rim combination measures differently, making a hard and fast “maximum tire” number impractical.

The actual measured width of any given tire can be significantly different than the labeled size. While it is generally close enough for a rule of thumb, a tire’s labeled size — “700x25” or “27.5x2.0” is at best an average and is primarily used as a marketing and sales metric for tire comparisons. It is not uncommon for a tire to be listed and sold as “700x38” and have an actual ERTO “40-622” marked on the tire, indicating 40mm width.

Another important consideration is the rim profile the tire is mounted on. Depending on variations in the internal rim width, the same tire can have noticeably different widths mounted on different wheels. It is not uncommon for a “700x28” tire to measure 28mm wide on some wheels and 33mm wide on others. In the example below, the same tire is 4mm wider on a “gravel/cyclocross” wheel with a 26mm internal width rim than on a “road/gravel” rim with 21mm internal width.

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Please see the specific frame/fork model page for maximum tire size guidelines, keeping in mind that those numbers are not universal.