John wrote:

Interesting, it got me thinking if the tires I have on now are really
the width that they claim to be. I measured them and found out they are
not. The 38mm is really only 34mm and the 42mm is really only 38mm
wide. Both are different make tires. Is this common for tires to be a
shorter width then they're marked for?

John

Measured width in bicycle tires is tricky. To some degree, the tire's
width depends on which rim the tire is mounted on. If the rim is on the
narrow side, the tire will measure narrower than its nominal width.

The marked width *should* be the width of the flattened-out casing,
measured bead-to-bead, divided by 2.5. I *think* this is the ISO/ETRTO
method of determining a given tire's width. I've never been able to
turn up a reference to this standard, though.

Of course, the manufacturer could simply be untruthful:
http://www.sheldonbrown.com/tire_sizing.html#dishonest

Jeff