You can try making your own version of this tire fit gauge from Rene Herse Cycles:
… Another way is to measure the distance along the chainstay from the rear dropout to where the tire would sit, and then measure the width between chainstays there. On a 650b wheel, the bead seat diameter (BSD) is 584 mm, which puts the radius at 584/2 = 292 mm. To that you would add half the tire width spec for where the widest part of the tire would end up, so basically you can measure out to: (584+tire width)/2, and then see how much space you have between chainstays at that point. The space between the chainstays should ideally allow at least about 5 mm on each side, so you end up having to provide 10 mm + Tire Width amount of space there.
Normally, in my experience, if the maximum 700c tire that a bike will fit is 28 mm, it’ll let you run 30-32 mm tires (true width). Continental, Rivendell, Hutchinson, and Rene Herse offer some tires in those widths, to varying degree of “feel”.
- Max “10 650b conversions down, two more to go?..” in A2