Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 35 Post(s)
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It's impossible for a manufacturer to ensure that a tire will inflate to it's nominal size, because he can't control the rim width which is part of the total cross-section circumference. That said, most manufacturers try to be somewhat close, based on an assumption of the intended rim width.
Manufacturers vary in their assumptions, and some tend to always be low, others tend to be high, but I'm surprised at the 15% difference in 2 tires from the same maker.
Unfortunately there's no 100% reliable to predict exact width but you can approximate it with a dollar bill. Wrap the bill around an uninflated tire bead to bead, and mark the distance, then add the inside rim width, and measure that distance. Divide by Pi (or 3 is close enough) to approximate inflated width.
This isn't a calibrated, precise way to get the width, but will give a fairly accurate prediction of the difference between two tire. Once you get used to it, and start compensating for tread, and where on the bead to start and finish, you'll get pretty decent predictions of how wide any tire will inflate to.
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