Originally Posted by
terrymorse
Yeah, measured width. I got that bit of learning recently, too. GP5k 25s now measure about 27.5mm on my new rims, and they're precariously close to rubbing on the chain stays. Also, the Silca calculators recommends substantially lower pressure for 27.5mm than 25mm.
The Silca calculator must derive rim width off the measured tyre width as there Is no input for rim width. The SRAM calculator uses rim width and nominal tyre width. But they seem to end up with the same result for my tyre and rim combo.