Making tire molds continues to be an inexact art. For decades, riders have complained that many tires, especially wider ones, do not run “true to size.” In most cases, tires were narrower than their stated width, which caused Sheldon Brown to suspect a plot by the tire makers: To make their tires appear lighter, tire manufacturers overstated the width, so that their 25 mm tires (labeled “28 mm”) were compared to the competition’s 28 mm tires, and so forth.
I don’t know whether this is true, but I suspect that the real reasons for the inaccurate sizes are less sinister: When tire makers design a mold, they don’t know exactly how wide the tires will be. This is especially true for relatively wide tires, and even more so for tires with supple casings. Simply changing the casing from the original Grand Bois Cypres 650B x 32 mm to the more supple (and much faster) “green label” casing increased the width of the tires by almost 2 mm – even though the mold was unchanged. In fact, because most tire makers have limited experience with wide high-performance tires, these tires often run wider than initially planned. This holds true for many Grand Bois tires, but also Challenge’s Paris-Roubaix and Michelin’s Pro Race models in the widest sizes.
We have decided to call our Grand Bois tires by the actual size, no matter what is written on the sidewalls. So we now offer:
700C x 23 mm (same as label)
700C x 26 mm (same as label)
700C x 29 mm (labeled as 28 mm)
700C x 32 mm (labeled as 30 mm)
650B x 32 mm (same as label)
650B x 38 mm (labeled as 36 mm)
650B x 42 mm (same as label)
Of course, even these measurements depend on the width of your rim. If you use wide rims, your tires can run a millimeter or so wider. Also remember that high-performance tires tend to “grow” a bit over the first few weeks of being mounted. In the end, a few millimeters should not matter much, as long as your bike had adequate clearances.
We are happy to offer Grand Bois tires in enough sizes now that most cyclists can enjoy their superior performance and comfort.