Originally Posted by
Retro Grouch
If you care enough to actually measure tires that have ben installed on a rim and inflated, you'll find that different width rims will result in differing tire width measurements.
Even with the same rim, you will see lots of variation from manufacturer to manufacturer.
OP...if you are riding your bike on anything other than flawlessly smooth pavement 100% of the time (i.e. a track), 21mm tires are a dumb idea.